The Treasures of Trappist-1 Script (Patreon)

Last week in a NASA press release, we learned about an amazing new discovery – a nearby red dwarf star was discovered to have not one, but seven Earth-like planets, and any of them may be capable of supporting life. 

The discovery was made by Dr Michael Gillon and team and reported in the journal Nature. The initial discovery of 3 planets in this system was made in 2015 with the Transiting Planets and Planetesimals Small Telescope – TRAPPIST – at La Silla Observatory in Chile. Follow-up with the Spitzer Space Telescope has now revealed an additional four. Both telescopes used the transit method - watching for the dimming of the central star as the planets pass in front of it. Never before have we seen so many Earth-sized planets in one place. Any one of them could bear liquid water, maybe even life.

The TRAPPIST-1 system lies 39 and a half lightyears away in the constellation of Aquarius. The star TRAPPIST-1A is an ultra-cool M-dwarf star, about 10% the Sun's diameter and less than 10% its mass. It's also much colder, about 2500 K compared to our sun’s 5800 K. The seven planets huddle extremely close to their star – all within one fifth of Mercury’s orbit. That makes a year on a TRAPPIST-1 planet very short, from a day and a half on the innermost TRAPPIST-1b to three weeks on TRAPPIST-1h. That’s a lot of birthday cake. The planets live so close together that they all tug on each other gravitationally. It’s enough to slightly vary the lengths of their years. Their gravitational influence on each other allows us to figure out their masses: between .4 and 1.4 times Earth’s mass. This, combined with the depth of the eclipses which gives their sizes, we can know their densities and so their compositions. They are very likely rocky or watery worlds like our own.

The planets have by now settled into stable orbit resonances with each other, and that’s a hint that they formed further out and then migrated inwards. It’s also unlikely that enough material resources would have been available for planet building so close to the star. Because different chemicals condense at different temperatures, planets’ distance from the star during formation largely determines the planets’ chemical makeup. If they did form further out then the TRAPPIST-1 septuplets may have a lot of water and other volatiles.

Maybe the TRAPPIST-1 planets started out as mixtures of rock and ice. However, three of them now occupy their star’s habitable zone, where planet surface temperature could be just right for liquid water. We can estimate the location of the habitable zone for a given star based on the intensity of its photon flux and the effect of atmospheric greenhouse gases. Our Solar System’s habitable zone extends from roughly one AU to one and a half AU, covering Earth and Mars. The same calculation for TRAPPIST-1 forms a thin band about two hundred times smaller, but in that densely packed-system it covers three of the planets. Gillon and team have done some climate modeling which indicate that the three inner-most planets may be more Venusian – overheated due to a runaway greenhouse effect – however that’s really hard to predict at this point. The outer-most planet, which orbits past the snow line, should be icy, however it may be that internal heat left over from formation and generated by tidal interactions with its star could warm it to liquid-water temperatures as well. In fact any of the seven planets could host liquid water.

Should we pack our bags and move to TRAPPIST-1? What would it be like there? We’ve already established the temperature could be to our liking. Wien’s law tells us that the 2500 Kelvin TRAPPIST-1 star shines brightest at infrared wavelengths. It would look pink on the sky. But there will be no day and night, so an eternal sunrise. See, because the planets are so close to the star they’re probably tidally locked, like our Moon. One side of each planet will always face the star, the other, away. That could be okay for habitability if they have sufficiently thick atmospheres to mix the heat from their sun globally. These planets would be huge in each others’ skies. I’m talking no-man’s-sky huge. At closest approach some will appear larger than a full moon – up to twice that size –  so they could easily see each other’s continents if they have them. Partial eclipses of the central star by sister planets may be common, but there would be no full eclipses. The TRAPPIS-1a star is just too close by, spanning a whopping five and a half degrees on the sky for if you're standing on the innermost planet, and a degree for on the outermost. For comparison, our sun and moon span around a half a degree on the sky. On the inner planet’s sunny side, the star will provide about as much visible light as our Sun. Planets further out will receive less light, but the star’s  infrared intensity provides the heat needed for liquid water.

Sounds pretty sci-fi awesome. But being so close to their central star exposes the planets  to stellar activity. Out here on Earth, our distance and magnetosphere protect us from most of our Sun’s coronal mass ejections. Within a tenth of an AU of TRAPPIST-1A, Coronal mass ejections would be dangerous, and tidal locking would weaken the planets’ protective magnetospheres. As a result, the TRAPPIST-1 winds may have eroded its planets atmospheres, just as the Sun did with Mars. In addition, although this star is now relatively quiet, when these M-dwarfs are young they are extremely active. This planetary system probably had a traumatic youth, which may not have been ideal for starting life. It’s not yet known how old the system is, so we don’t know if it’s had time to recover.

We don’t yet know much about what those atmospheres are like. Is it breathable air? Is it cyanide? All sorts of fascinating, nasty compounds can end up in exoplanets’ atmospheres, all strongly affecting climate, temperature, and chemistry. The Hubble Space Telescope has confirmed that the a, b, and h planets don’t have hydrogen-helium atmospheres, which means they aren’t gaseous like Jupiter. Future spectroscopy from the James Webb Space Telescope will hopefully provide us with atmospheric composition data. That may include signatures of chemicals indicative of the presence of biological activity. Of Life.

Finally, I mentioned tidal heating could warm TRAPPIST-1h above water’s freezing point. However, tidal forces could pose a serious problem to the inner planets. Picture Io, squished and stretched so vigorously by Jupiter and neighbor Europa that it’s a hellish, volcanic world. Tidal force between two bodies is proportional to the product of the masses and the inverse cube of their distance. There is so much more mass in the TRAPPIST-1 star-planet system than the Jupiter-Io-Europa system that, while the tides are about half the magnitude of Io’s on TRAPPIST-1h, they may be up to a hundred times as bad as Ios on TRAPPIST-1b. That could seriously increase temperatures and volcanic activity.

Clearly not all planets in the habitable zone are created equal. Liquid water is just Item One on a long list of requirements for life as we know it. Tidal locking, stellar activity, atmospheric composition, and volcanism can all turn an Earthlike planet uninhabitable. But this finding tells us that Earth-sized planets are probably common around -dwarf stars. Remember also that a potentially habitable Earth-like planet was recently found orbiting the very nearby proxima centauri red dwarf. Given that red dwarfs are the most numerous stars in the galaxy, we may have just seen a giant boost in the number of possible homes for life out there in space time.