The atmosphere of a planet holds the keys to make it habitable, so we need to look at them to figure if exoplanets are habitable. They are too far to send probes to measure them directly like we do with Mars or Jupiter’s moons, but scientists can study them from right here, looking at how they block light.
A couple of days ago, NASA announced the discovery of seven-count-them-seven rocky planets orbiting a small star called TRAPPIST1. Three of them seem to even be in the “habitable zone”. So, have we found the aliens’ home?
Nope. However, words are a bit confusing, so let’s review what it takes for a planet to be habitable.
The key is liquid water. All life we know—from bacteria to cats, from carrots to Lionel Messi—hinges on chemical reaction that only take place in water. No water, no life.
The habitable zone of a star is the space around it where planets can have sustained liquid water on their surface. Closer, and the heat from the star will fry the planet with all the water (think Mercury), farther and the surface will freeze (think Pluto).
Now we need a surface to collect this water on. That’s why rocky planets are interesting: no rocky surface, no place for water. And again, no water, no life.
Location isn’t everything, though: the atmosphere is key too. Earth’s atmosphere keeps water on the surface, and temperatures friendly (for planetary standards). Less of it and we’d risk turning into Mars, which is in the habitable zone, but is a frozen desert, where water is more like a killer sludge. More atmosphere, and we might become Venus (also in the habitable zone), which is effectively hell, molten lead lakes and deadly acid rains included. Winds in the atmosphere also favor habitability on some exoplanets.
Then you must hold on to your atmosphere. Earth has a cozy magnetic field that deflects part of the Sun’s radiation. Mars probably had an atmosphere, when it also had oceans, but has no magnetic field. Atom by atom, the constant barraging of energy and particles from the Sun eroded it. With the atmosphere gone, so was the water. No water, no life.
The planet discovered around TRAPPIST1 are important not because they might be habitable (though some are promising), but because TRAPPIST1 is the first star of its kind we study, and it immediately delivered several promising planets. That means it should be relatively easy to find interesting planets around that kind of star, which multiplies the chances of finding actually habitable ones.
Moreover, they are close to Earth (“just” 40 light years), so we can study their atmospheres with existing telescopes, or with the James Webb space telescope slated to launch next year. We’ll train on these for the multitude of planets we are about to find.
For now, as usual, no aliens.
If you want more
- Do you remember what a fuss was made that time NASA found the first Earth-sized planet in a habitable zone? This time they were three at once.
- NASA put together an impressive amount of information, graphics, and even an app. You can find everything here
- The real star of the bunch is this phenomenal 360 video (fullscreen highly recommended!) of the view standing on TRAPPIST1d
A long time ago Mars had water on its surface, and maybe even oceans–we knew that. But now we
‘re sure think that a little water still flows there. Sometimes. Kinda.
The Mars Reconnaissance Orbiter probe collected pictures of slopes, where long streaks stretch and shrink seasonally.
Because they conspicuously resemble small water streams, scientists studied them combining the great images from the HiRISE telescope on board the probe with spectroscopic measurements (which measure different light wavelengths to determine the chemical composition of a material).
The result is that these streaks (called Recurring Slope Lineae, or RSL) have all the makings of being caused by water flowing.
But RSLs are likely more similar to mud than water. Since Mars is really cold (-63 Celsius on average), the only way for water to prevent freezing is to have enormous concentrations of salt. The most probable candidate is perchlorate, which is almost everywhere on the planet surface. And it’s very toxic.
So don’t quite picture these as happy little mountain streams. They’re more like small avalanches of killer mud.
It’s also totally unclear where the water comes from. One possibility is that a thick layer of ice just below the surface thaws in the summer. Another is that there are actual underground waterbeds on Mars. Or maybe the water comes from the atmosphere, and the perchlorate captures it to the ground.
It may not look like much, but until now Earth was the only planet we knew with liquid surface water. This is a big step to figure how water works in the solar system.