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.
“One small step for (a) water bear, one giant leap for water-dwelling eight-legged segmented micro-animals.” —Teddy Tardigrade
Are you thinking what I’m thinking? Because if you are, you’re thinking that exposing tardigrades to high-energy cosmic rays can only mean one thing: super-tardigrades. From Live Science:
The Israeli spacecraft Beresheet crashed into the moon during a failed landing attempt on April 11. In doing so, it may have strewn the lunar surface with thousands of dehydrated tardigrades, Wired reported yesterday (Aug. 5). Beresheet was a robotic lander. Though it didn’t transport astronauts, it carried human DNA samples, along with the aforementioned tardigrades and 30 million very small digitized pages of information about human society and culture. However, it’s unknown if the archive — and the water bears — survived the explosive impact when Beresheet crashed, according to Wired.
Mindy Weisberger, Senior Writer
Well, OK, as tough as they are, it’s probably unlikely that those microscopic explorers will re-hydrate any time soon before being hit by high-energy particles that will then endow the tiny guys with Marvel-like superpowers, but it’s nice to dream.
But what are tardigrades? Let’s go back to Mindy’s Live Science article, because her explanation is simply too adorable not to reprint:
Tardigrades, also known as moss piglets, are microscopic creatures measuring between 0.002 and 0.05 inches (0.05 to 1.2 millimeters) long. They have endearingly tubby bodies and eight legs tipped with tiny “hands”; but tardigrades are just as well-known for their near-indestructibility as they are for their unbearable cuteness.
Moss piglets! Or should we now say moon piglets?
Light-hearted tardigiggles aside, it’s hard not to feel sorry for the tiny sleeping creatures. In a dehydrated state, they can remain hibernating (I’m not sure if that’s the correct term for being freeze-dried, but let’s go with hibernating) for a decade (!) while they wait for water to appear so they can go about their tardigradey business. They’ve been discovered in just about every environment on Earth, are extremely resilient and can even survive in space without a tiny spacesuit to keep them warm. In short, they’re pretty amazing. And now they’re on the Moon, which may or may not be a good thing (there’s a lot of cosmic rays up there).
Who doesn’t love the moon? You just have to look up when the skies are clear and there it is, our lunar friend, doing its thing, changing phases, yanking at our oceans, inspiring the world to look “up.”
It’s little wonder, then, that humanity has created many different names for our planet’s tidal partner in crime. There are useful astronomical names that describe its different phases (new/full, first/third quarter, waxing/waning crescent/gibbous), but there’s also other names that have popped up throughout human history that relate to other subtleties in the lunar dance around our world. A quasi-rare second full moon of the month? Blue moon! When the full moon coincides with perigee (lunar close approach with Earth)? Supermoon! When you get a bonus lunar combo that includes a full moon, a supermoon… and the Earth is blocking the sun so we have a lunar eclipse… and it all happens to occur in January?? That’s a SUPER BLOOD WOLF MOON ECLIPSE!Because of course it is.
As you may or may not have realized, humans—particularly humans in marketing departments, the media, and astrologers with too much time on their hands—like to label things. Some of these labels can be useful, others not so much. Many are, frankly, just plain silly. Which brings me to today’s lunar branding non-event: The Black Moon. Ohh sounds… eerie.
As one who has been involved in the broadcasting field for nearly 40 years, I’d like to point out that we live in a time when the news media is seemingly obsessed with “branding.” This marketing strategy involves creating a differentiated name and image — often using a tagline — in order to establish a presence in people’s mind. In recent years in the field of astronomy, for example, we’ve seen annular eclipses — those cases when the moon is too small to completely cover the disk of the sun — become branded as “Ring of Fire” eclipses. A total eclipse of the moon — when the moon’s plunge through the Earth’s shadow causes the satellite to turn a coppery red color — is now referred to as a “Blood Moon.”
When a full moon is also passing through that part of its orbit that brings it closest to Earth — perigee — we now brand that circumstance as a supermoon. That term was actually conjured up by an astrologer back in 1979 but quite suddenly became a very popular media brand after an exceptionally close approach of a full moon to Earth in March 2011. It surprises me that even NASA now endorses the term, although it seems to me the astronomical community in general shies away from designating any perigee full moon as “super.”
Then there is Blue Moon. This moniker came about because a writer for Sky & Telescope Magazine misinterpreted an arcane definition given by a now-defunct New England Almanac for when a full moon is branded “blue,” and instead incorrectly reasoned that in a month with two full moons, the second is called a Blue Moon. That was a brand that quietly went unnoticed for some 40 years, until a syndicated radio show promoted the term in the 1980s and it then went viral. So now, even though the second full moon in a month is not the original definition for a Blue Moon, in popular culture we now automatically associate the second full moon in a calendar month with a Blue Moon.
So are you ready for yet another lunar brand? The newest one is Black Moon.
That’s a very polite way of saying, “it’s all bullshit, really.”
So, what IS a Black Moon? Well, it’s the opposite of a Blue Moon, as in it’s the second New Moon in the month of July and a New Moon is when the sun, moon and Earth are in almost exact alignment; the entire Earth-facing side of the moon is in complete shadow. The upshot is you can’t see it. It’s a naked-eye astronomical non-event.
Having said that, should the moon exactly line up with the sun, you get a solar eclipse—arguably the most mind-blowing astronomical event we can see on Earth. A plain ol’ Black Moon? Not so much.
UPDATE: As this post turned into the seed for a fun little online discussion, I added some thoughts in the following Twitter thread. Feel free to @ me:
There’s nothing subtle about this deadly consequence of global warming.
While the recent record-breaking temperatures in Europe have grabbed the headlines, it’s worth remembering that such record-shattering heatwaves are nothing new to other regions of the planet. And many of those regions are fast approaching a grim reality: heat events that will overwhelm the body’s ability to function.
Once this wetbulb temperature threshold is crossed, the air is so full of water vapour that sweat no longer evaporates. Without the means to dissipate heat, our core temperature rises, irrespective of how much water we drink, how much shade we seek, or how much rest we take. Without respite, death follows – soonest for the very young, elderly or those with pre-existing medical conditions.
Wetbulb temperatures of 35°C have not yet been widely reported, but there is some evidence that they are starting to occur in Southwest Asia. Climate change then offers the prospect that some of the most densely populated regions on Earth could pass this threshold by the end of the century, with the Persian Gulf, South Asia, and most recently the North China Plain on the front line. These regions are, together, home to billions of people.
Tom Matthews, Climate Scientist, Loughborough University, The Conversation.
Matthews goes on to warn of “grey swan” events (read his research here, via Nature Climate Change), where overwhelming heat and moisture is coupled with mass power outages triggered by anthropomorphic global warming-boosted extreme weather events to leave vast populated regions physically unable to keep cool.
While many effects of climate change may seem subtle or “something for future generations to worry about,” this extreme situation will happen sooner rather than later, and as Matthews discusses, it has probably already been experienced.
Any debate about the realities of climate change is a distant dot in the rear-view mirror, and, according to a recent study, the scientific consensus that humans are driving global warming has passed 99 percent. (In reality, the consensus that humans are causing the planet to heat up has been an overwhelming majority for years, likely decades.)
Sadly, scientific consensus isn’t enough to stymie the emissions of greenhouse gasses—if it was, the oil rigs and coal mines would have been shut down years ago. It’s the human disposition for greed and myopic politics that will turn this once ecologically-diverse planet into an increasingly inhospitable place for humans to thrive.
The pushback has been political rather than scientific. In the US, the rightwing thinktank the Competitive Enterprise Institute (CEI) is reportedly putting pressure on Nasa to remove a reference to the 97% study from its webpage. The CEI has received event funding from the American Fuel and Petrochemical Manufacturers and Charles Koch Institute, which have much to lose from a transition to a low-carbon economy.
Policy makers who claim to be “skeptical” about the overwhelming scientific consensus that humans are causing global warming aren’t necessarily uneducated fools. They simply do not care. Democracy has long been hijacked by special interest groups and corporations that care little about the future health of the environment and society. In the long run, their belligerent self-interest will undercut their bottom line. It won’t be long until our carbon-driven economy will collapse under the weight of relentless impacts caused by the continued burning of fossil fuels.
It’s the ultimate self-own, and it’s a shame they’ll take us with them.
The pair of white dwarf stars are orbiting one another every seven minutes—and future gravitational wave observatories will be able to detect them whirl.
White dwarf binaries are among some of the most fascinating star systems known, and a newly discovered compact binary, located some 8,000 light-years away in the constellation Boötes, has taken the exotic nature of these systems to new spacetime-warping extremes.
Astronomers using Caltech’s Zwicky Transient Facility (ZTF), a precision sky survey at Palomar Observatory near San Diego in Southern California, made the discovery of ZTF J1530+5027 by detecting the dimming effect caused by one of the stars passing in front of the other. Known as an “eclipsing binary,” the cooler (and therefor dimmer) white dwarf blocks the starlight of the hotter (and brighter) star, causing the ZTF to register a periodic dimming event. This dimming occurs once every seven minutes, meaning they are zipping around each other at speeds of hundreds of miles per second! It is the second fastest white dwarf binary known and the most rapid eclipsing binary discovered in our galaxy. The fortunate alignment allows astronomers to not only precisely measure their orbital speed, they can also gauge the stars’ sizes and masses.
White dwarfs are the stellar corpses of sun-like stars that ran out of fuel long ago. Our sun will become a white dwarf in around five billion years, after it has exhausted its hydrogen fuel that maintains fusion in its core. A short period after, it will swell into a red giant (possibly expanding out as far as Earth, incinerating it) and then lose its plasma to space via powerful solar winds. All that will be left of our once glorious star will be a planetary nebula and a tiny and dense white dwarf, approximately the size of our planet, spinning in the middle. The two white dwarfs in ZTF J1530+5027 likely passed through their red giant phase at different times, but now they’re stuck, in a perpetual death spiral that spells doom for one of the objects.
To fully realize just how crazy-extreme this white dwarf binary is, they are only separated by one-fifth of the distance that the moon orbits Earth, meaning both stars would fully fit inside Saturn. They have a combined mass of our entire sun. As they orbit so snugly, it’s likely that the more massive star will start to tidally drag material from the other, cannibalizing it.
“Matter is getting ready to spill off of the bigger and lighter white dwarf onto the smaller and heavier one, which will eventually completely subsume its lighter companion,” said Kevin Burdge, Caltech graduate student and lead author of a study published in the journal Nature. “We’ve seen many examples of a type of system where one white dwarf has been mostly cannibalized by its companion, but we rarely catch these systems as they are still merging like this one.”
While impressive, the real fireworks are invisible—the stars are ripping up spacetime, generating gravitational waves that are sapping energy from the system, hastening the binary’s ultimate demise. What’s more, astronomers are anticipating that the future Laser Interferometer Space Antenna (LISA), which is scheduled for launch by the European Space Agency in 2034, will be able to detect its gravitational pulse.
“These two white dwarfs are merging because they are emitting gravitational waves,” added collaborator Tom Prince, a senior research scientist at Caltech and NASA’s Jet Propulsion Laboratory (JPL, in Pasadena, Calif. “Within a week of LISA turning on, it should pick up the gravitational waves from this system. LISA will find tens of thousands of binary systems in our galaxy like this one, but so far we only know of a few. And this binary-star system is one of the best characterized yet due to its eclipsing nature.”
This system is expected to keep blinking from our perspective for another hundred thousand years, but how will the system ultimately go kaput? Well, the researchers aren’t entirely certain. On the one hand, the more massive white dwarf may suck the other dry like a vampiric parasite, consuming all of its matter until only one, well-fed star remains. Alternatively, the act of cannibalization may cause the reverse; as mass is transferred to one star, the other may be flung outward to a wider orbit, increasing their orbital period.
“Sometimes these binary white dwarfs merge into one star, and other times the orbit widens as the lighter white dwarf is gradually shredded by the heavier one,” said co-author Jim Fuller, an assistant professor at Caltech. “We’re not sure what will happen in this case, but finding more such systems will tell us how often these stars survive their close encounters.”
One early mystery about this extreme binary is the question of X-rays, or lack thereof. The more massive star is really hot, with a temperature nine times that of the sun (50,000 Kelvin). The researchers believe that this is because it has already begun pulling material from its partner, an act that accelerates and heats the plasma that is being stolen, starting to create an accretion disk. But the accreting gas should be so hot that the system would be humming in X-rays, but it isn’t. “It’s strange that we aren’t seeing X-rays in this system. One possibility is that the accretion spots on the white dwarf—the areas the material is falling on—are bigger than what is typical, and this could result in the emission of ultraviolet light and optical light instead of X-rays,” added Burdge.
It’s exciting to think what the next generation of gravitational wave observatories (particularly LISA that will be sensitive to extremely weak spacetime ripples from systems such as these) combined with traditional (re: electromagnetic) observatories will herald for the future of astronomy. Like the emerging “multi-messenger” era for astronomy that combines observations of the electromagnetic spectrum and gravitational wave signals to confirm short gamma-ray bursts are triggered by neutron star collisions, it’s going to blow our minds when we can access more subtle gravitational wave sources such as these and directly see the gravitational energy leaking from compact binaries.
An international team of experts have teamed up to conclude that the interstellar visitor isn’t what we hoped it was.
It probably comes as no surprise that the scientific consensus of ‘Oumuamua’s origins have concluded that it is a natural object, despite how funky and alien spaceship-looking the interstellar visitor at first appeared. According to a new study published today in the journal Nature Astronomy, the findings of 14 international experts have been pooled to categorically say that ‘Oumuamua isn’t an artificial object piloted by an intelligent extraterrestrial species, but instead “has a purely natural origin.”
“The alien spacecraft hypothesis is a fun idea, but our analysis suggests there is a whole host of natural phenomena that could explain it,” said the team’s leader Matthew Knight, from the University of Maryland, in a statement.
This most recent study comes hot on the heels of a fair amount of speculation that the spinning cigar-shaped object, which was detected by the Pan-STARRS1 telescope in Hawaii on Oct. 19, 2017, could be artificial. One of the more vocal advocates of this possibility, Avi Loeb of Harvard University, investigated the idea that ‘Oumuamua may be an interstellar probe that used our sun’s radiation pressure for a boost in velocity as it flew through the inner solar system. While the world’s media loved this concept (as did I), many scientists balked and emphasized the need to take the Occam’s razor approach and instead focus on natural explanations, not aliens. But, as pointed out by Loeb, while more likely explanations existed, considering the most extreme ones is still a part of the scientific process.
“This is how science works,” said Loeb in an interview for The Harvard Gazette late last year. “We make a conjecture … and if someone else advances another explanation, we will compare notes and the next time we see an object of this type we will hopefully be able to tell the difference. That’s the process by which science makes progress.”
Deep down, we all had the sense that the interstellar visitor likely wasn’t aliens (though it did spawn some wonderful debates about mind-boggling interstellar distances, the challenges of visiting other star systems, and why ET would bother popping by for a whistle-stop tour without saying “hi”), but this new study convincingly sounds the death knell for the possibility of aliens taking a joyride through our galactic neighborhood.
The new study is clear, in which the researchers write: “Here we review our knowledge and find that in all cases, the observations are consistent with a purely natural origin for ‘Oumuamua.”
So, what does the study conclude?
The object is most likely an ancient interstellar comet that randomly encountered our solar system after drifting through interstellar space for millions of years. The mechanisms by which ‘Oumuamua was ejected from its star system of birth remains up for debate, but the study’s authors point to the likelihood of a Jupiter-like world that may have gravitationally ejected the object when it strayed too close, helping it achieve escape velocity and a future lost deep in the interstellar expanse—until it encountered our solar system.
Even the behavior of the ancient comet as it traveled through the inner solar system agrees with theoretical predictions. The small boost in velocity as it made close approach to our sun was caused by ices (entombed under ‘Oumuamua’s surface) being heated and vented into space as a vapor (and not aliens hitting the gas). This behavior in comets is well-known, but the problem with ‘Oumuamua is that it exhibited few signs of being a comet—it didn’t develop a tail nor did it develop a coma, two clues of its cometary nature. But this object is different from the comets we know; it has been drifting through the galaxy for eons, perhaps it lost the majority of its ice in previous stellar encounters, or perhaps it contained little in the way of volatiles during its formation. Comets and asteroids also have a lot more in common that the textbooks may tell us, so perhaps it did vent small quantities of vapor to give it a boost, but not enough for astronomers to observe a tail and coma. In short, ‘Oumuamua shares similar traits to other objects that exist in our solar system
“While ‘Oumuamua’s interstellar origin makes it unique, many of its other properties are perfectly consistent with objects in our own solar system,” added Robert Jedicke of the University of Hawai’i’s Institute for Astronomy (IfA) and collaborator in the Nature Astronomy study.
The key thing that makes ‘Oumuamua so captivating, however, is not how it behaved when it entered the solar system and used the sun to change its course, it’s that we know it came from interstellar space, the first of its kind that we’ve ever encountered. Undoubtedly, the solar system has been visited countless times by junk that has been shed by other stars in our galaxy—there’s a lot of stars carrying around a lot of comets and asteroids, after all, they’re probably scattered around the Milky Way like baby’s toys being thrown out of strollers—but this is the first, special interstellar visitor that we’ve only just had the ability to detect.
The best news? There will be more.
Humanity is rapidly advancing through a “golden age” for astronomy and, if these interstellar vagabonds are as common as we now believe, we’re on the verge of detecting many more of them. For example, the Large Synoptic Survey Telescope (LSST), which is being constructed in Chile, is expected to become operational in 2022 and it will be so powerful that astronomers predict at least one ‘Oumuamua-like object will be spotted per year. Once we grasp how often these things turn up, perhaps we’ll be prepared enough to have a robotic spacecraft intercept one to see what these visitors from other stars really look like instead of depending on distant observations.
Of course, this whole episode could be a cautionary tale. Perhaps our advanced alien neighbors disguise their spacecraft to look like passing comets to get a closer look of primitive intelligences such as ourselves.* ‘Oumuamua being identified as an interstellar comet is exactly what they want us to believe…
*This was inspired by a tweet I read this morning, but I forgot who tweeted it and it appears I didn’t “like” it, so it’s since been lost to the twitterverse. Thank you to whomever tweeted it, it formed the seed to this blog!
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.
The two GRAIL spacecraft flew one in front of the other, precisely measuring the distance of their separation in order to detect very small fluctuations in the Moon’s gravitational field. When the spacecraft passed over a region of higher density, the local gravitational field would become enhanced, slightly accelerating the leading spacecraft (called “Ebb”) before the trailing spacecraft (“Flow”) experienced that acceleration. By mapping these acceleration fluctuations, scientist have gained an invaluable understanding of density fluctuations deep below the Moon’s surface that would have otherwise remained invisible.
During this recent analysis, the researchers discovered a gravitational “anomaly” beneath the South Pole-Aitken basin—a vast depression on the lunar far side spanning 2,000 miles wide and several miles deep. This anomaly represents a huge accumulation of mass hundreds of miles below the basin.
“Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That’s roughly how much unexpected mass we detected,” said Peter B. James, of Baylor University and lead author of the study, in a statement.
How did all that material end up buried inside the Moon’s mantle? The South Pole-Aitken basin was created four billion years ago in the wake of a massive asteroid impact. In fact, the basin is known to be one of the biggest impact craters in the solar system. If this crater was formed by an impact, it stands to reason that the gravitational anomaly is being caused by the dense metallic remains of the massive asteroid that met its demise when the Earth-Moon system was in the throes of formation.
“When we combined [the GRAIL data] with lunar topography data from the Lunar Reconnaissance Orbiter, we discovered the unexpectedly large amount of mass hundreds of miles underneath the South Pole-Aitken basin,” added James. “One of the explanations of this extra mass is that the metal from the asteroid that formed this crater is still embedded in the Moon’s mantle.”
There may be other explanations, one of which focuses on the formation of the Moon itself. As the lunar interior cooled after formation, the large subsurface mass could be an accumulation of “dense oxides associated with the last stage of lunar magma ocean solidification,” the researchers note.
The metallic corpse of an ancient asteroid is the leading candidate, however, and computer simulations carried out by the team indicated that if the conditions are right, the dense iron-nickel core of an asteroid can be dispersed inside the Moon’s mantle where it remains embedded today without sinking into the lunar core.
Although there were certainly larger asteroid impacts throughout the history of our solar system, the Moon’s South Pole-Aitkin basin is the largest preserved impact crater known, making it a prime candidate to study ancient impact sites
“[It’s] one of the best natural laboratories for studying catastrophic impact events, an ancient process that shaped all of the rocky planets and moons we see today,” said James.
It just so happens that we currently have a mission at the basin, exploring this strange and unexplored place. On Jan. 3, the Chinese Chang’e 3 mission achieved the first soft touchdown on the lunar far side, landing inside Von Kármán crater and releasing a robotic rover, Yutu-2, to explore the landscape. At time of writing, the mission is ongoing.
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.