It’s both too hot and too cold, has no atmosphere, and is no place to take a vacation—but there is an upside.
It’s hard to say anything positive about the exoplanet LHS 3844b. It’s a wretched place; an alien world that orbits its tiny star in less than half a day. As it’s so close to its red dwarf star, it’s tidally-locked—when one side of the planet is always in baking daylight, the other side is in a perpetual frozen night. Oh, and it doesn’t even have an atmosphere.
Why the heck am I even writing about this unfortunate celestial object?
Well, it might not be our idea of an interstellar getaway, but it is remarkable for two profound reasons: It’s a rare look at the surface conditions of a rocky exoplanet orbiting a distant star, and the very fact that astronomers are confident it doesn’t have an atmosphere is a really big deal.
World of Extremes
Discovered in 2018, LHS 3844b is located nearly 49 light-years away. It has a radius 30 percent larger than Earth and orbits a cool M dwarf star. It was detected by NASA’s newest space-based exoplanet hunter, the Transiting Exoplanet Satellite Survey (TESS); every 11 hours, the world drifts in front of the star, blocking a tiny amount of light (and event known as a “transit”) that can be detected by TESS. As it orbits so close to its host star, it’s glowing bright in infrared radiation, giving the researchers of a new study published in Nature an incredible opportunity.
Using observational data from NASA’s Spitzer space telescope, which views the universe in infrared wavelengths, and as the star is comparatively cool and dim, the researchers could discern how much infrared radiation was being emitted from the exoplanet’s “day” side and calculated that it must be cooking at a temperature of 1,410 degrees Fahrenheit (770 degrees Celsius). On measuring the infrared emissions from the exoplanet’s dark side, they realized that the heat from the day side wasn’t being transported to the night side. On Earth, our atmosphere distributes thermal energy around the globe, ensuring that the night and day sides’ temperature difference isn’t so extreme. LHS 3844b, however, isn’t distributing any of its thermal energy creating a sharp drop-off in temperature between both hemispheres. In other words: no atmosphere!
“The temperature contrast on this planet is about as big as it can possibly be,” said Laura Kreidberg, a researcher at the Harvard and Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and lead author of the new study. “That matches beautifully with our model of a bare rock with no atmosphere.
“We’ve got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven’t been able to study them empirically. Now, with LHS 3844b, we have a terrestrial planet outside our solar system where for the first time we can determine observationally that an atmosphere is not present.”
This exoplanet has about as much atmosphere as the planet Mercury or our Moon, and it shares some other traits too. By measuring the amount of starlight the exoplanet reflects (a characteristic known as “albedo”), Kreidberg’s team also took a stab at understanding its composition.
As the world is “quite dark,” they deduced that it’s very likely that it’s covered in basalt (volcanic rock), the same stuff that we find in the crusts of the Moon and Mercury. “We know that the mare of the Moon are formed by ancient volcanism,” said Renyu Hu, an exoplanet scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., “and we postulate that this might be what has happened on this planet.”
An Atmospheric Problem
Red dwarfs are known to play host to entire systems of exoplanets, including many small rocky worlds of similar dimensions as Earth. Many of these worlds have been found within the much-hyped “habitable zone”, where it’s neither too hot or too cold for liquid water to persist. As we probably are all aware, liquid water is super helpful for life (on our planet, at least) to evolve. While LHS 3844b could never be considered “habitable” in any way, shape or form, the fact that it doesn’t have an atmosphere may be very revealing.
It’s simply not good enough to find a habitable zone exoplanet that orbits a red dwarf and say “there’s a good chance that aliens live there!” Even though it has the right temperature, because it’s orbiting a red dwarf and likely tidally-locked could mean these types of worlds are devoid of atmospheres, a serious wrench in the hope that all habitable zone exoplanets have the same likelihood of life. That’s not to say all red dwarf-orbiting exoplanets lack atmospheres, but now at least we are developing techniques that could, one day, help us from the atmospheric potential of more “habitable” candidates.
While it’s an exciting discovery, the nearby star system is a very alien place with its own unique array of challenges.
The universe is stranger than we can imagine, so when a star system is discovered with some familiar traits to ours, it can be hard not to imagine extraterrestrial lifeforms and interstellar getaways. But before you dream of bathing on the exotic shores of Teegarden b, breathing in the moist and salty air, while sipping on a Teegarden Tequila Sunrise, keep in mind that the reality will likely be, well, much stranger than we can imagine.
All of these facts are cause for celebration, no? They are, but a heavy dose of reality needs to be applied when it comes to any world that has been discovered beyond our solar system.
More Exoplanets, More Possibilities
As alien planet-hunting missions continue to add more worlds to the vast menagerie of known exoplanets that exist in our galaxy, an increasing number of them are falling inside the “habitable zone” category.
The habitable zone around any star is the distance at which a rocky planet can orbit where it’s neither too hot or too cold for liquid water to exist on its surface (if it has water, that is). Liquid water is the stuff that Earth-like biology has an affinity to; without it, life on Earth wouldn’t have evolved. So, even before we have any clue about its H2O-ness, if an exoplanet is seen to have an orbit around its star that is deemed habitable, that’s +1 point for habitability.
Now, the next point can only be won if that world is also of approximate Earth-like size and/or mass. There would be little reason in getting too excited for a Jupiter-sized exoplanet sitting in the habitable zone possessing liquid water on its “surface” (because it won’t have a surface). That’s not to say there can’t be some gas giant-dwelling balloon-like alien living in there, but we’re looking for Earth-like qualities, not awesome alien qualities we read in science fiction. (I’d also argue that these kinds of exoplanets might have habitable Earth-sized moons—like Avatar‘s Pandora—but that’s for another article…)
The two key methods for exoplanet detection is the “radial velocity” method and the “transit” method. The former—which precisely measures a star’s light to detect tiny stellar wobbles as an exoplanet gravitationally “tugs” at it as it orbits—can deduce the exoplanet’s mass, thereby revealing whether or not it has an Earth-like mass (Teegarden’s two worlds were discovered using this method). The latter—which was employed by NASA’s Kepler space telescope (and now NASA’s Transiting Exoplanet Survey Explorer, among others) to look for the slight dips in brightness as an exoplanet passes in front of its star—can deduce the exoplanet’s physical size, thereby revealing whether or not it has an Earth-like size. Should a habitable zone exoplanet possess either one of these Earth-like qualities, or both (if both methods are used on a target star), that’s another +1 point for its habitability.
There’s a few other measurements that astronomers can make that may add to a hypothetical world’s habitability (such as observations of the host star’s flaring activity, age, or some other derived measurement), but until we develop more powerful observatories on Earth and in space, there are several factors that quickly cause our hypothetical exoplanet to diminish in habitable potential.
The Unhabitability of “Habitable” Worlds
So far in our burgeoning age of exoplanetary studies, we’ve only been able to measure (and derive) a handful of characteristics—such as mass, orbital period, physical size, density—but we have very little idea about these habitable zone exoplanets’ atmospheres. Apart from measurements of a few massive and extreme exoplanets—such as “hot-Jupiters” and exoplanets getting blow-torched by their star when they venture too close—astronomers haven’t been able to directly measure the existence of any of these “habitable” exoplanet’s hypothetical atmospheres. Do they even possess atmospheres? Or are they the opposite, with hellish Venus-like pressure-cooker atmospheres? Who knows. Even if they do have atmospheres that are more Earth-like, are the gases they contain toxic to life as we know it?
Recently, theoretical models of exoplanetary atmospheres brought carbon dioxide and carbon monoxide into the discussion. CO2 is a powerful greenhouse gas that helps maintain a balance in our atmosphere, regulating a temperate world (until industrialized humans came along, that is). But too much can be a very bad thing. For exoplanets existing on the outer edge of their habitable zone to remain habitable, they’d need massive concentrations of CO2 to remain temperate—concentrations that would render the atmosphere toxic (to complex lifeforms, at least). In the case of carbon monoxide (the terrible gas that asphyxiates anything with a cardiovascular system), as our star is so hot and bright, its ultraviolet radiation destroys large accumulations of CO in Earth’s atmosphere. But for habitable zone exoplanets that orbit cool red dwarf stars (like Teegarden), huge concentrations of CO may accumulate and snuff-out life before it has the opportunity to evolve beyond a germ. These two factors are a big negative against life as we know it, shrinking the effective habitable zone around certain stars and certain exoplanetary orbits.
Most habitable zone exoplanets have been found orbiting red dwarfs, primarily because our observations have been biased in favor of these little stars—they’re small and cool, meaning that any planet orbiting within their habitable zones need to get up-close and personal, so it’s an easier task to detect the periodic star wobbles or exoplanetary transits to confirm their existence.
While this may sound cute, orbiting so close to a red dwarf is a blessing (for astronomers) and a curse (for any unfortunate aliens). Many red dwarf stars generate powerful stellar flares that would regularly bombard nearby worlds with radiation that terrestrial biology would not be able to tolerate. Unless those planets have incredibly powerful global magnetic fields to, a) protect their inhabitants from being irradiated and, b) prevent the savage stellar winds from stripping away their protective atmospheres, there’s limited hope for the evolution of life.
Interestingly, however, according to the Teegarden study published in the journal Astronomy & Astrophysics, this particular red dwarf is relatively quiet on the life-killing flare front, so that’s something. Another tentative +1 for Teegarden’s actual habitability!(Pass the tequila.)
As you can tell, there’s lots of exciting implications balanced by plenty of sobering reality checks. There is, however, one factor that is often missed from big announcements about worlds orbiting small stars that, whether they are habitable or not, is truly beyond our experience.
Eyeballing Temperate Red Dwarf Systems
Teegarden is an eight-billion-year-old star system, approximately twice the age of our solar system. If life has found a way, it will have come and gone, or be in an evolved state (though this is anyone’s guess, we have little idea about the hows and whys of the emergence of life on Earth, let alone on a different planet). But the worlds themselves, if either possess liquid water (Teegarden b, being the one that should be the most temperate of the pair, so will have the higher odds), they certainly wouldn’t look like Earth, even if they have Earth-like qualities.
Having settled billions of years ago, any orbital instabilities would have ebbed, and the planetary orbits would be clearly defined and likely in some kind of resonance with the other bodies in the star system. In addition, both Teegarden b and c will, in all likelihood, be tidally locked with their star.
To understand what this means, we need only look up. When we see our moon, we only see one hemisphere—the “near side”; the lunar “far side” is never in view. Except for the Apollo astronauts, no human has ever seen the moon’s far side with their own eyes. That’s because the moon’s rotation period (28 days) exactly matches its orbital period (28 days) around the Earth. Other examples of tidally-locked systems in the solar system are Pluto and its largest moon Charon, Mars and both its moons Phobos and Diemos, plus a whole host of moons orbiting Jupiter, Saturn, Uranus and Neptune.
The same tidal physics applies to red dwarf stars and their closely-orbiting worlds. And Teegarden b and c have very close orbits, zipping around the star once every five and eleven days, respectively, so they are very likely tidally locked, too.
So what does a habitable zone exoplanet orbiting a red dwarf star look like? Enter the “Eyeball Earth” exoplanet:
I’ve written about this hypothetical world before and it fascinates me. As temperate exoplanets orbit red dwarfs so snugly, and if they have an atmosphere, they may too look like the above artistic rendering.
Looking like an eyeball, the star-facing hemisphere of the planet will be perpetually in daylight, whereas the opposite side will be in perpetual night. The near-side will likely be an arid desert, but the far side will be frozen. Computer simulations of the atmospheric dynamics of such a world are fascinating and well worth the read. The upshot, however, is that these worlds may have dynamic atmospheres where habitability is regulated by powerful winds that blast from the star-facing hemisphere to the night-side, transporting water vapor in a surprisingly complex manner. These worlds will never be fully-habitable, but they may host in interesting array of biological opportunities nonetheless.
For example, there may be a “ring ocean” that separates the desert from the ice, where, on one side, tributaries flow into the hot hemisphere only to be evaporated by the incessant solar heating. The vapor is then transported anti-star-ward, only to be deposited as it freezes on the night-side. One could imagine this massive buildup of ice on the planets night-side as an hemisphere-wide glacier that slowly creeps sun-ward, where it melts and pools into a temperate ring ocean where the process starts all over again.
Like Earth, the atmospheric dynamics would need to be balanced perfectly and if an alien ecosystem manages to get a foothold, perhaps such a planet-wide “water cycle” could be sustained while maintaining the life that thrives within.
So, whenever we hear about the latest exoplanetary discovery, and take note that these strange new worlds are “Earth-like” or “habitable,” it’s worth remembering that neither may be accurate. Sure, finding an Earth-sized world in orbit around their star in the habitable zone is a great place to start, but it’s just that, a start. What about its atmosphere? Does it have the right blend of atmospheric gases? Is it toxic? Does it even have an atmosphere? Whether or not an alien world has a global magnetic field could make or break its habitable potential. Does its star have sporadic temper tantrums, dousing any local planets with a terrible radiation storm?
These challenges are no stranger to the astronomers who find these worlds and speculate on their astrobiological potential, but in the excitement that proceeds the discovery of “Earth-like” and “habitable” exoplanets, the headlines are often blind to the mechanics of what really makes a world habitable. The next step will be to directly observe the atmospheres of habitable exoplanets, a feat that may be within reach when NASA’s James Webb Space Telescope (JWST) and the ESO’s Extremely Large Telescope (ELT) go online.
The fact is, we know of only ONE habitable world, all the others are hypothetically habitable—so let’s look after this one while it can still sustain the rich and diverse ecosystem we all too often take for granted.
Don’t forget your spacesuit: Complex lifeforms, such as humans, would not survive on many of the worlds we thought would be interstellar tropical getaways
Worlds like Earth may be even rarer than we thought.
We live on a planet that provides the perfect balance of ingredients to support a vast ecosystem. This amazing world orbits the Sun at just the right distance where water can exist in a liquid state—a substance that, as we all know, is an essential component for our biology to function. Earth is also an oddball in our solar system, being the only planet where these vast oceans of liquid water persist on its surface, all enshrouded in a thick atmosphere that provides the stage for a complex global interplay of chemical and biological cycles that, before we industrialized humans came along, has supported billions of years of uninterrupted evolution and biological diversity.
Humans, being the proud intelligent beings that we profess to be, are stress-testing this delicate balance by pumping an unending supply of carbon dioxide into the atmosphere. Being a potent greenhouse gas, we’re currently living through a new epoch in our planet’s biological history where an exponential increase in CO2 is being closely followed by an increase in global average temperatures. We are, in effect, altering Earth’s habitability. Well done, humans!
While this trend is a clear threat to the sustainability of our biosphere, spare a thought for other “habitable” worlds that may appear to have all the right stuff for complex lifeforms to evolve, but toxic levels of the very chemicals that keep these worlds habitable has curtailed the possibility of complex life from gaining a foothold.
Welcome to the Not-So-Habitable Zone
Habitable zone exoplanets are the Gold Standard for exoplanet-hunters and astrobiologists alike. Finding a distant alien world within this zone—a region surrounding any star where it’s not too hot and not too cold for water to exist on its surface, a region also known as the “Goldilocks Zone” for obvious reasons—spawns a host of questions that our most advanced telescopes in space and on the ground try to answer: Is that exoplanet Earth-sized? Does it have an atmosphere? What kind of star is it orbiting? Does its system possess a Jupiter-like gas giant? These questions are all trying to help us understand whether that world has the Earthly qualities that could support hypothetical extraterrestrial life.
(Of course, there’s the debate as to whether all life in the universe is Earth-life-like, but as we’re the only biological examples that we know of in the entire galaxy, it’s the best place to start when pondering what biological similarities extraterrestrial life may have to us.)
The habitable zone for exoplanets is a little more complicated than simply the distance at which they orbit their host stars, however. Greenhouse gases, such as carbon dioxide, can extend the area of a star’s habitable zone. For example: If an atmosphere-less planet orbits beyond the outermost edge of its habitable zone, the water it has on its surface will remain in a solid, frozen state. Now, give that planet an atmosphere laced with greenhouse gases and its surface may become warm enough to maintain the water in a liquid state, thereby boosting its habitable potential.
But how much is too much of a good thing? And how might this determination impact our hunt for truly habitable worlds beyond our own?
In a new study published in the Astrophysical Journal, researchers have taken another look at the much-coveted habitable zone exoplanets to find that, while some of the atmospheric gases are essential to maintain a temperature balance, should there be too much of the stuff keeping some of those worlds at a habitable temperature, their toxicity could curtail any lifeforms more complex than a single-celled microbe from evolving.
“This is the first time the physiological limits of life on Earth have been considered to predict the distribution of complex life elsewhere in the universe,” said Timothy Lyons, of the University of California, Riverside, and director of the Alternative Earths Astrobiology Center.
“Imagine a ‘habitable zone for complex life’ defined as a safe zone where it would be plausible to support rich ecosystems like we find on Earth today,” he said in a statement. “Our results indicate that complex ecosystems like ours cannot exist in most regions of the habitable zone as traditionally defined.”
Carbon dioxide is an essential component of our ecosystem, particularly as it’s a greenhouse gas. Acting like an insulator, CO2 absorbs energy from the Sun and heats our atmosphere. When in balance, it stops too much energy from being radiated back out into space, thereby preventing our planet from being turned into a snowball. Levels of CO2 have ebbed and flowed throughout the biological history of our planet and it has always been a minor component of atmospheric gases, but its greenhouse effect (i.e. the atmospheric heating effect) is extremely potent and the human-driven 400+ppm levels are causing dramatic climate changes that modern biological systems haven’t experienced for millions of years. That said, the CO2 levels required to keep some “habitable” exoplanets in a warm enough state would need to be a lot more concentrated than the current terrestrial levels, potentially making their atmospheres toxic.
“To sustain liquid water at the outer edge of the conventional habitable zone, a planet would need tens of thousands of times more carbon dioxide than Earth has today,” said lead author Edward Schwieterman, of the NASA Astrobiology Institute. “That’s far beyond the levels known to be toxic to human and animal life on Earth.”
From their computer simulations, to keep CO2 at acceptable non-toxic levels, while maintaining planetary habitability, the researchers realized that for simple animal life to survive, the habitable zone will shrink to no more than half of the traditional habitable zone. For more complex lifeforms—like humans—to survive, that zone will shrink even more, to less than one third. In other words, to strike the right balance between keeping a hypothetical planet warm enough, but not succumbing to CO2 toxicity, the more complex the lifeform, the more compact the habitable zone.
This issue doesn’t stop with CO2. Carbon monoxide (CO) doesn’t exist at toxic levels in Earth’s atmosphere as our hot and bright Sun drives chemical reactions that remove dangerous levels of the molecule. But for exoplanets orbiting cooler stars that emit lower levels of ultraviolet radiation, such as those that orbit red dwarf stars (re: Proxima Centauri and TRAPPIST-1), dangerous levels of this gas can accumulate. Interestingly, though CO is a very well-known toxic gas that prevents animal blood from carrying oxygen around the body, it is harmless to microbes on Earth. So it may be that habitable zone exoplanets orbiting red dwarfs could be a microbial heaven, but an asphyxiation hell for more complex lifeforms that have cardiovascular systems.
While it could be argued that life finds a way—extraterrestrial organisms may have evolved into more complex states after adapting to their environments, thereby circumventing the problems complex terrestrial life has with CO2 and CO—if we are to find a truly “Earth-like” habitable world that could support human biology, these factors need to be considered before declaring an exoplanet habitable. And, besides, we might want to make the interstellar journey to one of these alien destinations in the distant future; it would be nice to chill on an extraterrestrial beach without having to wear a spacesuit.
“Our discoveries provide one way to decide which of these myriad planets we should observe in more detail,” said Christopher Reinhard, of the Georgia Institute of Technology and co-leader of the Alternative Earths team. “We could identify otherwise habitable planets with carbon dioxide or carbon monoxide levels that are likely too high to support complex life.”
Earth: Unique, Precious
Like many astronomical and astrobiological studies, our ongoing quest to explore strange, new (and habitable) worlds has inevitably led back to our home and the relationship we have with our delicate ecosystem.
“I think showing how rare and special our planet is only enhances the case for protecting it,” Schwieterman said. “As far as we know, Earth is the only planet in the universe that can sustain human life.”
So, before we test the breaking point of our atmosphere’s sustainability, perhaps we should consider our own existential habitability before its too late to repair the damage of carbon dioxide emissions. That’s the only way that we, as complex (and allegedly intelligent) lifeforms, can continue to ask the biggest questions of our rich and mysterious universe.
The space telescope has refined the stellar flybys of the Voyager and Pioneer probes—how might it help us chart our way to the stars in the future?
When looking up on a starry night, it can be difficult to comprehend that those stars are not fixed in the sky. Sure, on timescales of a human lifetime, or even the entirety of human history, the stars don’t appear to move too much. But look over longer timescales—tens of thousands, to millions of years—and it becomes clear that the stars in the sky are in motion. This means the constellations we see today will be misshapen (or even non-existent!) in a few hundred thousand years’ time.
This poses an interesting question: If humanity were to send a spacecraft on an interstellar mission—an endeavor that could take thousands of years, depending on how ambitious the target—aiming it directly at a distant star would be a mistake. Depending on how far away that star is, by the time the spacecraft reaches its target, the star could have moved a few light-years away. This is why precision astrometry—the astronomical measurement of a star’s position, speed and direction of motion—will be needed to predict where a target star will be, and not where it currently is, when our future interstellar mission gets there.
To test this, we don’t need to wait until humanity has the means to build a starship, however. We have a bunch of interstellar probes that have already started their epic sojourns into the galaxy.
Earlier this year, NASA’s Voyager 2 spacecraft departed the Sun’s sphere of influence and became humanity’s second interstellar mission, six years after its twin, Voyager 1, made history to become the first human-made object to drift into the space between the stars. Both Voyagers are still transmitting telemetry to this day, over 40 years since their launch. Another two spacecraft, the older Pioneer 10 and 11 missions, are also on their way to interstellar space, but they stopped transmitting decades ago. A newcomer, NASA’s New Horizons mission, will also become an interstellar mission in the future, but it has yet to finish its Kuiper belt explorations and still has fuel to make course corrections, so predictions of its stellar encounters will remain unknown for some time.
Having explored the outer planets in the 1970’s and 80’s, the Voyagers and Pioneers barreled on, revealing stunning science from the outer solar system. In the case of Voyager 1 and 2, when each breached the heliopause (the invisible boundary that demarks the limit of the Sun’s magnetic bubble, between the heliosphere and interstellar medium), they gave us a profound opportunity to experience this distant alien environment, using their dwindling number of instruments to measure particle counts and magnetic orientation.
But where are our intrepid interstellar interlopers going now? With the help of precision astrometry of local stars observed by the European Space Agency’s Gaia space telescope, two researchers have taken a peek into the future, seeing which star systems the spacecraft will drift past in the next few hundred thousand to millions of years.
Previously, astronomers have been able to combine the spacecrafts’ trajectory with stellar data to see which stars they will fly past, but in the wake of the Gaia Data Release 2 (GDR2) last year, an unprecedented trove of information has been made available for millions of stars in the local galaxy, providing the most precise “road map” yet of those stars the Voyagers and Pioneers will encounter.
“[Gaia has measured] the positions and space velocities of nearby stars more precisely than before and so has more precisely characterized the encounters with stars we already knew about,” says astronomer Coryn Bailer-Jones, of the Max Planck Institute for Astronomy in Heidelberg, Germany.
Close Encounters of the Voyager Kind
Bailer-Jones and colleague Davide Farnocchia of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., published their study in Research Notes of the American Astronomical Society, adding another layer of understanding about where our spacecraft, and the stars they’ll encounter, are going. Although their work confirms previous estimates of some stellar close encounters, Bailer-Jones tells Astroengine.com that there have been some surprises in their calculations—including encounters that have not been identified before.
For example, the star Gliese 445 (in the constellation of Camelopardalis, close to Polaris) is often quoted as being the closest encounter for Voyager 1, in approximately 40,000 years. But with the help of Gaia, which is giving an extra layer of precision for stars further afield, the researchers found that the spacecraft will come much closer to another star, called TYC 3135-52-1, in 302,700 years.
“Voyager 1 will pass just 0.30 parsecs [nearly one light-year] from that star and thus may penetrate its Oort cloud, if it has one,” he says.
This is interesting. Keep in mind that the Voyager and Pioneer spacecraft include the famous Golden Records and plaques (respectively), revealing the location, form, and culture of a civilization living on a planet called “Earth.” For an alien intelligence to stumble across one of our long-dead spacecraft in the distant future, the closer the stellar encounter the better (after all, the likelihood of stumbling across a tiny spacecraft in the vast interstellar expanse would be infinitesimally small). Passing within one light-year of TYC 3135-52-1 is still quite distant (for instance, we currently have no way of detecting something as dinky as a Voyager-size probe zooming through the solar system’s Oort Cloud), but who knows what the hypothetical aliens in TYC 3135-52-1 are capable of detecting from their home world?
Another interesting thought is that these Gaia observations can help astronomers find stars that are currently very far away, but now we know their speed and direction of travel, some of those stars will be in our cosmic backyard in the distant future.
“What our study also found, for the first time, is some stars that are currently quite distant from the Sun will nonetheless come very close to one of the spacecraft within the next few million years,” says Bailer-Jones. “For example, the star Gaia DR2 2091429484365218432 is currently 159.5 parsecs [520 light-years] from the Sun (and thus from Voyager 1), but Voyager 1 will pass within 0.39 parsecs [1.3 light-years] of it in 3.4 million years from now.”
In some cases, given unlimited time, you may not have to go to a star, the star will come to you!
Our Interstellar Future?
While pinpointing the various stellar encounters for our first interstellar probes is interesting, the observations being made by Gaia will be important for when humanity develops the technology to make a dedicated effort to travel to the stars.
“It will be essential to have extremely precise astrometry of any target star,” explains Bailer-Jones. “We must also measure its velocity and its acceleration precisely, because these affect where the star will be when the spacecraft arrives.”
Although this scenario may seem a long way off, any precision astrometry we do now will build our knowledge of the local stellar population and boost the “legacy value” of Gaia’s observations, he adds.
“Once a target star has been selected, we would want to make a dedicated campaign to measure its position and velocity even more precisely, but to determine the accelerations we need data measured at many time points over long periods (at least tens of years), so Gaia data will continue to be invaluable in the future,” Bailer-Jones concludes.
“Even now, astrometry from the previous Hipparcos mission—or even from surveys from decades ago or photometric plates 100 years ago!—are important for this.”
Update (May 23): One of the reasons why I focused on the Voyager missions and not the Pioneers is because the latter stopped transmitting a long time ago. Another reason is because we already know Pioneer 10 doesn’t make it very far into interstellar space:
I, for one, welcome our new Mars desert-dwelling overlords.
It’s a question I’ve been pondering for some time: if we discover microbes eking out an existence on Mars, what might they be called? At first, I presumed it would be a variation on how we designate microbial names on Earth. Something like Staphylococcus aureus but swap out the “aureus” for “ares” (Greek for “Mars”, the god of war) or … something.
As you can see, biology isn’t my strong suit and butchering Latin and Greek is all in a day’s work. So, feeling out of my depth, I decided to leave that thought alone and file the idea under “Interesting, But Needs More Research.” That’s where the topic stayed for a while; I wanted to wait for a related piece of science to appear in a journal that could be a catalyst for my question. And last week, that research surfaced. I saw my opportunity.
Searching for Martians on Earth
The Atacama Desert is an amazing place. Having visited the ESO’s Paranal Observatory and the Atacama Large Millimeter/submillimeter Array in 2016 as a lucky member of the #MeetESO team, I have first-hand experience of that extreme and breathtaking region. While driving between sites, we’d often go for hours without seeing any vegetation or life of any kind. Atacama is the driest place on Earth; its salty, parched soil is bombarded by ultraviolet radiation, and the core of the desert doesn’t receive rain for decades. But just because life isn’t obvious in the arid ‘scapes, that doesn’t mean it’s not there.
The flora and fauna that does call Atacama their home are very specialized in finding ways to thrive. On the smallest life scales, for some microbes that means living underground, which makes them very interesting organisms indeed.
The research was driven, in part, to develop techniques for robotic missions to the Red Planet that will seek out alien bacteria that may be holed up in an underground colony. Remember, Mars has the same land area as Earth, so there’s a lot of real estate to search for microscopic lifeforms. Sure, scientists are smart and can narrow down potentially-habitable regions that they can drop a life-seeking robot on, but once landed on that toxic soil, what kind of methodology should they use to look for these hypothetical bacteria? The Atacama Desert makes for a decent analog of Mars; it’s very dry and its soil is laced with toxic perchlorate salts, so if microbes on Mars bear any resemblance to the nature of microbes in the Atacama, scientists can take a stab at predicting their behavior and guide their Mars rovers to the most likely places where they might be hiding.
Researchers already know that bacterial life occupies even the harshest Atacama regions, but according to team leader Stephen Pointing, a professor at Yale-NUS College in Singapore, the microbes we are familiar with are common species that live on the surface, using sunlight for energy. But Pointing isn’t so interested in what’s on the surface; his rover is fitted with a drill and extraction system that can take samples of soil from underground. During the campaign, Pointing’s team made some compelling discoveries.
“We saw that with increasing depth the bacterial community became dominated by bacteria that can thrive in the extremely salty and alkaline soils,” he told me. “They in turn were replaced at depths down to 80 centimeters by a single specific group of bacteria that survive by metabolizing methane.”
These subsurface microbes are known to science — they have been found in deep mine shafts and other subterranean environments — but they’ve never been found living under the surface of the world’s most arid region. They’ve also fine-tuned their evolution to specifically adapt to this harsh environment. “The communities of bacteria that we discovered were remarkably lacking in complexity, and this likely reflects the extreme stress under which they develop,” said Pointing.
The biggest discovery made during this research was that the subsurface colonies of bacteria were very patchy, said Pointing, a factor that will have ramifications for the search for their Martian cousins. “The patchy nature of the colonization suggest that a rover would be faced with a ‘needle in a haystack’ scenario in the search for Martian bacteria,” he said.
Desert Planet Survivor
This research is a fascinating glimpse into how Earth-based environments are being used to better understand how alien bacteria may evolve in their native environments. But the desert-thriving, methane-munching bacteria of the Atacama may also inspire their name — should they be discovered one day.
Pointing explained: “The way we assign Latin names to bacteria is based on their evolutionary relationship to each other and we measure this using their genetic code. The naming of Martian bacteria would require a completely new set of Latin names at the highest level if Martian bacteria were a completely separate evolutionary lineage — that is they evolved from a different common ancestor to Earth bacteria in a “second genesis” event [and not related to Earth life via panspermia]. If we find truly “native” Martian bacteria I would love to name one, and call it Planeta-desertum superstes, which translates in Latin to ‘survivor on the desert planet.'”
So there we have it, an answer to my question about what our Martian neighbors might be called, if we find them: Planeta-desertum superstes,the desert planet survivor.
Now that Opportunity’s mission is complete, many wistfully lament about “bringing our robot home.” There’s just one problem: it’s already home.
I am fascinated with how we anthropomorphize robots, particularly space robots. We call them “brave,” “pioneers” and even give them genders — usually a “she.” We get emotional when they reach the end of their missions, saying they’ve “died” or, as I like to say, “gone to Silicon Heaven.” But these robots are, for all intents and purposes, tools. Sure, they expand the reach of our senses, allowing us to see strange new worlds and parts of the universe where humans fear to tread, but they’re an assembly of electronics, metal, plastic, sensors, transmitters, wheels and solar panels. They don’t have emotions. They don’t breathe. They don’t philosophize about the incredible feats of exploration they are undertaking. They don’t have genders.
Still, we fall in love. When watching Curiosity land on Mars from NASA’s Jet Propulsion Laboratory, I teared up, full of joy that the six-wheeled hulk of a rover — that I’d met personally in JPL’s clean room a couple of years before — had safely landed on the Red Planet. After watching NASA’s InSight lander touch down on Elysium Planitia, again via JPL’s media room last year, there it was again, I was in love. I’m already anthropomorphizing the heck out of that mission, seeing InSight’s landing as another “heartbeat” on Mars. When the European Rosetta mission found Philae lying on its side like a discarded child’s toy on the surface of comet 67P/Churyumov–Gerasimenko, I jumped up from my desk with joy. When Cassini’s mission at Saturn ended in 2017, I was miserable. When the Chinese rover Yutu rolled off its lander in 2014, I realized I was cheering the robot on. When Spirit got stuck in a sand trap in Gusev Crater, I set up a Google alert for any and all news on the recovery efforts.
These emotions aren’t just for the exciting science and engineering strides humanity makes, there’s a certain inspirational character that each robot brings. Undoubtedly, this character naturally emerges from the wonderful scientists and engineers who design and build these amazing machines, and the social media managers who often “speak” for their robots in first person. But if you strip away the science, the technology and the people who build them, we still personalize our beloved robots, giving them their own character and creating a cartoon personality. I believe that’s a beautiful trait in the human condition (except a few flawed cultural and stereotypical missteps) and can be used to great effect to captivate the general public with the science that these robots do.
So there’s no great surprise about the outpouring of emotion for last week’s announcement that NASA called off the communications efforts with Mars Exploration Rover Opportunity. This kick-ass robot traveled 28 miles and lasted nearly 15 years, until a global dust storm in early 2018 starved it of sunlight. It landed on Mars way back in 2004, with its twin, Spirit, beginning its Martian reign with a hole-in-one, literally — after bouncing and rolling across the regolith after its entry and descent, encased inside a genius airbag system, it plopped inside the tiny Eagle crater. We’ve collectively lived through Opportunity’s adventures and the groundbreaking science it has done. There’s a huge number of terrific robot obituaries out there, so I won’t duplicate those efforts here. There is, however, a recurring sentiment that is somewhat misplaced, though entirely innocent.
Opportunity — like Spirit and all the Mars rovers and landers that have come and gone — died at home.
This may sound like an odd statement, but there seems to be this fascination with “returning” our space robots to Earth. I’ve seen cartoons of the Dr Who traveling through time to “rescue” Opportunity. People have argued for the case of future Mars astronauts returning these artifacts to terrestrial museums. There’s that touching XKCD cartoon of Spirit being “stranded” on Mars after NASA declared it lost in 2010, that is being resurfaced for Opportunity. We want our dusty Mars rover back!
It’s understandable, that rover has been continuously exploring Mars for a decade and a half, many of its fans, including myself, could check in on Opportunity’s adventures daily, browsing the latest batch of raw images that were uploaded to the NASA servers. We love that thing. In the tradition of military service members who die abroad, we go to great efforts to bring their bodies home so they can repatriated; we want to repatriate our science service member back to Earth.
But Opportunity is a robot that was designed for Mars. Every single design consideration took the Martian environment into account. The Red Planet’s gravity is roughly 1/3rd that of Earth, so the weight on its actuators and chassis are 2/3rds less than what they’d experience on our planet. Its motors are too under powered to reliably drive the robot forward on Earth. On Mars, they’re perfect. Granted, the mass of the Mars Exploration Rovers (approximately 185 kg) are a lot less than their supersized cousin, Curiosity (899 kg), but if Opportunity and Spirit had a 90-day mission exploring the dunes of the Californian Mojave Desert, I’m betting they wouldn’t get very far; they would be under-powered and grind to a halt. They’d also likely overheat as they were designed to withstand the incredibly low temperatures on the Martian surface.
The robots we send to Mars are undeniably Martian. If we’re going to anthropomorphize these beautiful machines, let’s think about what they’d want. I’m guessing they’d want to stay on that dusty terrain and not return to the alien place where they were constructed. And, in doing so, they become the first generation of archaeological sites on the Red Planet that, one day, the first biological Martians will visit.
A tiny rock has been detected in the Kuiper belt, which may not seem like such a big deal, but how it was found is.
We think we have a pretty good handle on how planets form. After the birth of a star, big enough clumps of dust and rock in the disk of leftover debris begin to accrete mass until they turn into spheres under the pull of their own gravity, jostling around, pushing smaller protoplanets out of the way and being shoved aside by, or smashing, into larger ones. Whatever planets survive this messy process end up becoming a solar system. We’ve seen this around other stars and aside from a few interesting twists on this model, we think we know what’s going on pretty well by now.
But there was one piece missing. The math says that to start the planet building process, you need a kind of planetary seed between one and ten kilometers wide. Since we happen to live in a solar system, we should be able to look outwards, towards the Kuiper Belt, which we think is made primarily from the leftovers of planetary formation, and see these protoplanetary fossils drifting across the sky. However, the process has proven to be rather tricky. These rocks are very faint and rather small compared to everything else we can usually see, so looking for them is kind of like trying to spot a grain of dust in a room illuminated only by moonlight, which is why we have so much trouble finding them.
Or at least we did until now, when a 1.3 kilometer Kuiper Belt Object, or KBO was spotted by a simple setup and commercially available cameras as it eclipsed background stars. While that might not sound like much right now, it’s actually an extremely important finding. First, it tells us how to find tiny KBOs so we can take a proper survey of protoplanetary leftovers. Secondly, it shows that we’re correct in our solar system formation model and demonstrated that predicted artifacts of baby planets that never quite made it do exist. The next part will be to try and detect more of these little planet seedlings to figure out how efficient the formation process is, and see what we can learn from that.
As noted, these finds don’t just apply to our own solar system, but to pretty much every planet in the universe. Just consider that mighty gas giants with swirling storms that could swallow Earth whole, exotic icy dwarfs with percolating cryovolcanoes and towering peaks dusted with reddish organic molecules, and tropical worlds with deep oceans teeming with life — which might even be home to an alien civilization living through its heyday — all started out as these little rocks lucky enough to clump together for a few hundred million years, find a stable orbit, and cool down enough to become a cosmic petri dish. They might not be impressive or exciting on their own, but that doesn’t mean they aren’t profoundly important.
Reference: Arimatsu, K., et. al., (2019) A kilometre-sized Kuiper belt object discovered by stellar occultation using amateur telescopes, Nature Astronomy Letters, DOI: 10.1038/s41550-018-0685-8
New models trying to infer the geology of potentially habitable moons orbiting Jupiter and Saturn hint at surprisingly cool, geologically inactive worlds, the opposite of what a diverse alien ecosystem would need
Imagine a spaceship finally landing on Europa and slowly drilling into the ice. After weeks of very careful progress, it pierces the moon’s frozen shell and releases a small semi-autonomous submarine connected to the probe with an umbilical to ensure constant communication and a human taking over in case of an emergency. Much of the time, it will chart a course of its own since piloting it with an hour long delay between command and response would be less than ideal. It navigates through the salty ocean, shining its light on structures never before seen by a human eye, making its way deeper and further into the alien environment to find absolutely… nothing at all.
That’s the sad scenario proposed by a team of geologists who crunched the numbers on the four leading contenders to host alien life in our outer solar system: Europa, Ganymede, Titan, and Enceladus. According to their models, looking at gravity, the weight of water and ice on the rocks underneath, and the hardness of the rocks themselves, these moons would be more or less geologically dead. Without volcanoes or sulfur vents, there would be very little in terms of nutrient exchange and therefore, very little food and fuel for an alien ecosystem more complex than microbe colonies.
Of course, these results are a pretty serious departure from the hypotheses commonly held by planetary scientists that the gravity of gas giants cause tidal kneading inside their moons, citing Io as an example. According to the researchers’ model, only Enceladus would be a promising world to look for life, as evidenced by the plumes breaking through its icy crust, spraying organic material into space. The reason why the numbers are different, they say, is because its core is likely to be porous, meaning its ocean would be heated deep inside the moon, fueling geysers and churning organic matter while effectively making the little world a ball of soggy slush.
Since these findings are so different from what’s implied by observations, the researchers aren’t in a rush to publish them are are soliciting other scientists’ opinions to make sure they have a complete picture, and lead investigator Paul Byrne grumbled about his disappointment with what the models indicate. That said, while he’s hoping to be proven wrong, we shouldn’t forget that these are alien worlds and while we’ve spent decades studying them, our knowledge came in bursts. Simply put, we might know a fair bit but far from everything and disappointing surprises may lurk under their icy surfaces and subterranean oceans.
The binary system observed by ALMA isn’t wonky, it’s the first example of a polar protoplanetary disk
Some star systems simply don’t like conforming to cosmic norms. Take HD 98000, for example: It’s a binary system consisting of two sun-like stars and it also sports a beautiful protoplanetary disk of gas and dust. So far, so good; sounds pretty “normal” to me. But that’s only part of the story.
When a star is born, it will form a disk of dust and gas — basically the leftovers of the molecular cloud the star itself formed in — creating an environment in which planets can accrete and evolve. Around a single star (like our solar system) the protoplanetary disk is fairly well behaved and will create a relatively flat disk around the star’s spin axis. For the solar system, this flat disk would have formed close to the plane of the ecliptic, an imaginary flat surface that projects out from the sun’s equator where all the planets, more or less, occupy. There are “wonky” exceptions to this rule (as, let’s face it, cosmic rules are there to be broken), but the textbook descriptions of a star system in its infancy will usually include a single star and a flat, boring disk of swirling material primed to build planets.
Cue HD 98000, a star system that has flipped this textbook description on its head, literally. As a binary, this is very different to what we’re used to with our single, lonely star. Binary stars are very common throughout the galaxy, but HD 98000 has a little something extra that made astronomers take special note. As observed by the Atacama Large Millimeter/sub-millimeter Array (ALMA), its protoplanetary disk doesn’t occupy the same plane as the binary orbit; it’s been flipped by 90 degrees over the orbital plane of the binary pair. Although such systems have been long believed to be theoretically possible, this is the first example that has been found.
“Discs rich in gas and dust are seen around nearly all young stars, and we know that at least a third of the ones orbiting single stars form planets,” said Grant M. Kennedy, of the University of Warwick and lead author of the study published today in the journal Nature Astronomy, in a statement. “Some of these planets end up being misaligned with the spin of the star, so we’ve been wondering whether a similar thing might be possible for circumbinary planets. A quirk of the dynamics means that a so-called polar misalignment should be possible, but until now we had no evidence of misaligned discs in which these planets might form.”
This star system makes for some rather interesting visuals, as shown in the artist’s impression at the top of the page. Should there be a planetary body orbiting the stars on the inner edge of the disk, an observer would be met with a dramatic pillar of gas and dust towering into space with the two stars either side of it in the distance. As they orbit one another, the planetary observer would see them switch positions to either side of the pillar. It goes without saying that any planet orbiting two stars would have very different seasons than Earth. It will even have two different shadows cast across the surface.
“We used to think other solar systems would form just like ours, with the planets all orbiting in the same direction around a single sun,” added co-author Daniel Price of Monash University. “But with the new images we see a swirling disc of gas and dust orbiting around two stars. It was quite surprising to also find that that disc orbits at right angles to the orbit of the two stars.”
Interestingly, the researchers note that there are another two stars orbiting beyond the disk, meaning that our hypothetical observer would have four suns of different brightnesses in the sky.
The most exciting thing to come out of this study, however, is that ALMA has detected signatures that hint at dust growth in the disk, meaning that material is in the process of clumping together. Planetary formation theories suggest that accreting dust will go on to form small asteroids and planetoids, creating a fertile enviornment in which planets can evolve.
“We take this to mean planet formation can at least get started in these polar circumbinary discs,” said Kennedy. “If the rest of the planet formation process can happen, there might be a whole population of misaligned circumbinary planets that we have yet to discover, and things like weird seasonal variations to consider.”
What was that I was saying about “cosmic norms”? When it comes to star system formation, there doesn’t appear to be any.
A nearby baby star has been discovered with a warped protoplanetary disk — a feature that may reveal the true nature of the solar system’s planetary misalignments
Textbook descriptions of our solar system often give the impression that all the planets orbit the sun in well-behaved near-circular orbits. Sure, there’s a few anomalies, but, in general, we’re led to believe that everything in our interplanetary neighborhood travels around the sun around a flat orbital plane. This, however, isn’t exactly accurate.
Pluto, for example, has an orbit around the sun that is tilted by over 17 degrees out of the plane of the ecliptic (an imaginary flat plane around which the Earth orbits the sun). Mercury has an inclination of seven degrees. Even Venus likes to misbehave and has an orbital inclination of over three degrees. If all the material that built the planets originated from the same protoplanetary disk that was — as all the artist’s impressions would have us believe — flat, what knocked all the planet’s out of alignment with the ecliptic?
Until now, it was assumed that, during the early epoch of our solar system’s planet-forming days, dynamic chaos ruled. Planets jostled for gravitational dominance, Jupiter bullied smaller worlds into other orbits (possibly chucking one or two unfortunates into deep space), and gravitational instabilities threw the rest into disorderly orbital paths. Other star systems also exhibit this orbital disorder, so perhaps it’s just an orbital consequence of a star system’s growing pains.
But there might be another contribution to the chaos: perhaps wonky star systems were just born that way.
Cue a recent observation campaign of the nearby baby star L1527. Located 450 light-years away in the direction of the Taurus Molecular Cloud, L1527 is a protostar embedded in a thick protoplanetry disk. Using the Atacama Large Millimeter/submillimeter Array (ALMA), in Chile, astronomers of the RIKEN Cluster for Pioneering Research (CPR) and Chiba University in Japan discovered that the L1527 disk is actually two disks morphed into one — both of which are out of alignment with one another. Imagine a vinyl record that has been left on a heater and you wouldn’t be far off visualizing what this baby star system looks like.
The RIKEN study, published on Jan. 1 in Nature, suggests that this warping may have been caused by jets of material emanating from the star’s birth, kicking planet-forming material into this warped configuration and, should this configuration remain stable, could result in planets with orbital planes that are significantly out of alignment.
“This observation shows that it is conceivable that the misalignment of planetary orbits can be caused by a warp structure formed in the earliest stages of planetary formation,” said team leader Nami Sakai in a RIKEN press release. “We will have to investigate more systems to find out if this is a common phenomenon or not.”
It’s interesting to think that if this protoplanetary disk warping is due to the mechanics behind the formation of the star itself, we might be able to look at mature star systems to see the ancient fingerprint of a star’s earliest outbursts or, possibly, its initial magnetic environment.
It’s possible “that irregularities in the flow of gas and dust in the protostellar cloud are still preserved and manifest themselves as the warped disk,” added Sakai. “A second possibility is that the magnetic field of the protostar is in a different plane from the rotational plane of the disk, and that the inner disk is being pulled into a different plane from the rest of the disk by the magnetic field.”
Though orbital chaos undoubtedly contributed to how our solar system looks today, with help of this research, we may be also getting a glimpse of how warped our sun’s protoplanetry disk may have been before the planets even formed.