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!
The space exploration industry is booming, which is an encouraging sign for our future. But some pundits are arguing that rocket launches will exacerbate global warming.
When so many people, especially those in charge, seem so cavalier about the impact of global warming and climate change on our planet, it’s refreshing to see a perspective that worries about what we’re doing to our environment. Unfortunately, when that perspective focuses on a tiny contributor and seems to lack the understanding of what it criticizes, it needs to be called out. A number of pundits looked at the exploding private space industry and have grown concerned that rocket launches we will inject too much greenhouse gas into the atmosphere, exacerbating global warming and the attendant problems that come with it. And while it’s true that rocket fuel is far from clean, releasing plenty of unwanted chemicals into the atmosphere as it burns, we have to keep the big picture in mind.
When it comes to launching things into space, there aren’t that many alternatives to rockets and their toxic fuel. You can’t use an ion drive or any of the other seemingly sci-fi but realistic propulsion methods for traveling to other worlds and solar systems. Earth’s gravity and atmospheric pressure at sea level are very different from the vacuum of the cosmos where the tiniest push can really add up in the long term. The only way to get tons of supplies and machinery into orbit and beyond is through controlled explosions harnessed by rockets. There is simply no other way currently feasible, and there won’t be until we figure out how to build giant electromagnetic railguns, or how to harness antimatter, although that would come with a high risk of exposure to gamma radiation.
We could conceivably launch human crews in single stage to orbit planes, but their spacecraft are going to have to rely on good old-fashioned rocketry. That said, however, the plan is not to simply keep launching things from earth with no regard to the pollution thousands of rockets launched every year would cause. Launching payloads from Earth is expensive, both financially and energetically, so ideally, we would want to launch them from somewhere else. We would want to take off from the Moon or asteroids, somewhere where the gravity is in a fraction of what it is on our world, and we could use the same engines to propel anywhere between six and a hundred times the cargo. This is what we mean by infrastructure for space exploration. Forget about turning Earth into a giant launchpad. The ideal gateway to the rest of the solar system is the Moon.
Lacking an atmosphere, the Moon doesn’t particularly care how toxic the fuel is or how much greenhouse gas each launch produces. For all intents and purposes, the moon is a harsh and the radioactive wilderness with no environment to conserve. The same goes for asteroids we want to use as refueling stations, which are simply chunks of radiation-battered rock and metal floating through space we could harvest for fuel and building materials by using, of all things, steam powered asteroid-hopping robots. So, while it’s understandable to worry about the carbon footprint of everything that we do, considering the current inaction by so many on pressing climate issues, it’s important to keep things in perspective when doing so. If global warming continues apace, it won’t be thanks to rockets. It will be thanks to stubborn clinging to fossil fuels across the world and pollution from heavy industry and manufacturing.
If we were to push for serious investments in green energy, which is thankfully something that’s already happening, rocket launchers wouldn’t even be a blip on our carbon radar. Before we start asking ourselves how much carbon dioxide a SpaceX Falcon Heavy releases, and how many greenhouse gases it saves by reusing its booster cores, we need to ask ourselves how many coal plants are still powering cities and why, and what it will take to switch them over to clean, renewable sources. Otherwise, we’re doing the equivalent of trying to pay off the national debt by scrimping and saving on how many pencils public school teachers are allowed to get from their school districts. Which would be a funny analogy if it wasn’t true.
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.
The world’s most powerful radio telescope is getting intimate with Sagittarius A*, revealing a never-before-seen component of its accretion flow
As we patiently wait for the first direct image of the event horizon surrounding the supermassive black hole living in the core of our galaxy some 25,000 light-years away, the Atacama Millimeter/submillimeter Array (ALMA) has been busy checking out a previously unseen component of Sagittarius A*’s accretion flow.
Whereas the Event Horizon Telescope (EHT) will soon deliver the first image of our supermassive black hole’s event horizon, ALMA’s attention has recently been on a cool flow of gas that is orbiting just outside the event horizon, before being consumed. (The EHT delivered its first historic image on April 10, not of the supermassive black hole in our galaxy, but of the gargantuan six-billion solar mass monster in the heart of the giant elliptical galaxy, Messier 87, 50 million light-years away.)
While this may not grab the headlines like the EHT’s first image (of which ALMA played a key role), it remains a huge mystery as to how supermassive black holes pile on so much mass and how they consume the matter surrounding them. So, by zooming in on the reservoir of material that accumulates near Sagittarius A* (or Sgr A*), astronomers can glean new insights as to how supermassive black holes get so, well, massive, and how their growth relates to galactic evolution.
While Sgr A* isn’t the most active of black holes, it is feeding off limited rations of interstellar matter. It gets its sustenance from a disk of plasma, called an accretion disk, starting immediately outside its event horizon—the point at which nothing, not even light, can escape a black hole’s gravitational grasp—and ending a few tenths of a light-year beyond. The tenuous, yet extremely hot plasma (with searing temperatures of up to 10 million degrees Kelvin) close to the black hole has been well studied by astronomers as these gases generate powerful X-ray radiation that can be studied by space-based X-ray observatories, like NASA’s Chandra. However, the flow of this plasma is roughly spherical and doesn’t appear to be rotating around the black hole as an accretion disk should.
Cue a cloud of “cool” hydrogen gas: at a temperature of around 10,000K, this cloud surrounds the black hole at a distance of a few light-years. Until now, it’s been unknown how this hydrogen reservoir interacts with the black hole’s hypothetical accretion disk and accretion flow, if at all.
ALMA is sensitive to the radio wave emissions that are generated by this cooler hydrogen gas, and has now been able to see how Sgr. A* is slurping matter from this vast hydrogen reservoir and pulling the cooler gas into its accretion disk—a feature that has, until now, been elusive to our telescopes. ALMA has basically used these faint radio emissions to act as a tracer as the cool gas mingles with the accretion disk, revealing its rotation and the location of the disk itself.
“We were the first to image this elusive disk and study its rotation,” said Elena Murchikova, a member in astrophysics at the Institute for Advanced Study in Princeton, New Jersey, in a statement. “We are also probing accretion onto the black hole. This is important because this is our closest supermassive black hole. Even so, we still have no good understanding of how its accretion works. We hope these new ALMA observations will help the black hole give up some of its secrets.” Murchikova is the lead author of the study published in Nature on June 6.
Located in the Chilean Atacama Desert, ALMA is comprised of 66 individual antennae that work as one interferometer to deliver observations of incredible precision. This is a bonus for these kinds of accretion studies, as ALMA has now probed right up to the edge of Sgr A*’s event horizon, only a hundredth of a light-year (or a few light-days) from the point of no return, providing incredible detail to the rotation of this cool disk of accreting matter. What’s more, the researchers estimate that ALMA is tracking only a minute quantity of cool gas, coming in at a total only a tenth of the mass of Jupiter.
A small quantity this may be (on galactic scales, at least), but it’s enough to allow the researchers to measure the Doppler shift of this dynamic flow, where some is blue-shifted (and therefore moving toward us) and some is red-shifted (as it moves away), allowing them to clock its orbital speed around the relentless maw of Sgr A*.
“We were able to shed new light on the accretion process around Sagittarius A*, which is a typical example of a class of black holes that have little to eat,” added Murchikova in a second statement. “The accretion behavior of these black holes is quite complex and, so far, not well understood.
“Our result is potentially important not only for our galaxy, but to any galaxy which has this type of underfed black hole in its heart. We hope that this cool disk will help us uncover more secrets of black holes and their behavior.”
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: