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TRAPPIST-1: a star with three habitable planets, just 40 light years away

Scientists from MIT, University of Liege, and elsewhere, have found not one, not two, but three planets orbiting a single star, all of which seem to be habitable by a variety of measures. The team is calling them the best candidates yet found for life outside our solar system, and since they’re only 40 light years from Earth, they ought to be perfectly positioned for detailed further investigation. If you’re betting on which system out there is most likely to produce evidence of alien life, this one might be a good one to remember: 2MASS J23062928-0502285, also known as TRAPPIST-1.

The star is a so-called brown dwarf star, or a star that isn’t massive enough to exert the level of gravity needed to jump-start hydrogen fusion at its core. This means two things: It’s very cold (sometimes referred to as an ultra-cool dwarf star) and it doesn’t put out very much visible light. A regular star is, of course, a big lightbulb in the dark, meaning that when you stare right into it with a telescope, it tends to blind you; this is one of the main reasons it took so long to actually see exoplanets. Eventually, astronomers built customized planet-hunters meant specifically to stare into suns, and quickly found hundreds, then thousands of exoplanets. These sightings are known as “transits,” where the orbiting planet moves between the target star and the telescope, dimming the star for as long as the planet remains in the way.

2MASS puts out far less light to blow out our telescopes, but it’s actually so dark in visible light that few telescopes could really investigate it, or systems like it. A Belgian-funded telescope in Chile was created specifically to address this problem, of looking at very dim objects: the TRAnsiting Planets and PlanetesImals Small Telescope (TRAPPIST) — and yes, they did just use that I in the acronym. 2MASS J23062928-0502285, thus, actually has a different nickname: TRAPPIST-1. TRAPPIST was able to look at the brown dwarf star and measure the infra-red transits that other telescopes could never have captured. In 2010, its ability to look at very dim objects revealed some new information about the Kuiper belt dwarf planet Makemake, which was just last week revealed to have its own moon.

The readings from TRAPPIST 1 show three planets, all of which are roughly Earth-sized, and seem well within the habitable range on a number of axes. Two of the planets have just 1.5- and 2.3-day orbits (years) yet despite being so close, the brown dwarf is so cool that planetary surface temperatures would remain below the boiling point of water, though perhaps only slightly. They would each receive only a couple of times more radiation from their sun than the Earth receives from our Sun, which means there could be low enough level of harmful radiation to allow roughly genetic processes to develop.

The third planet orbits much further out, with an orbit around the star lasting between four and 73 days (hey, the readings are still quite new…), meaning that it may receive even less radiation than the Earth. It should still be warm enough to theoretically support life, however.

Since the TRAPPIST-1 star lies so close to Earth, astronomers should be able to start quickly figuring out more nuanced attributes of the planets, from their exact masses to the compositions of their atmospheres. This is what could take the planets from potentially habitable to probably habitable — though whatever life sprung up in that alien environment would be very different than the life we know on Earth.

Not only would its entire ecosystem have to run off of mostly infrared light, but it the TRAPPIST readings also suggest that all three new planets are what’s called “tidally locked” to their host star — a fact that implies we might actually be more likely to someday find life on hypothetical moons of these three planets. Tidal locking is the process that slowly adjusted the rotation of the Moon until it only ever showed us a single side, and in this case it means one side of each planet is in perpetual day, the other in perpetual night.

Either side will be a tough sell for life, though some estimates say a tidally locked system like this could maintain areas with liquid water on the dark side. In any case, we might imagine life well suited to exploiting the sharp gradients at the planet-encircling interface of light and shadow. Tidal locking on the moons of these planets, however, might benefit life since it produces heat through a process called tidal acceleration, which could keep the moons hot enough to support life and potentially develop an atmosphere.

The results were published in Nature today, and are sure to set off a storm of interest in the TRAPPIST-1 system. Right now, they’re just candidates for habitable planets — but since they’re so close and available, the rest of the info we need ought not to be far out at all. And since brown dwarfs are themselves very common, if they turn out to be good candidates for hosting habitable planets, then the search for life elsewhere in the universe could veer sharply toward the infra-red portion of the spectrum, and the dimmest systems in the sky.

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