Around every star there is a region where the frigid abyss of space is balanced by the searing heat of nuclear fusion in such a way that water can flow as a liquid. For our particular star, this region is roughly centered between Earth and Mars. Mars, as you might know, is much too cold for abundant liquid water. Earth, on the other hand, is covered in water in the form of oceans, clouds, and glaciers. In our current understanding of how life forms, liquid water is a crucially important. Watery Earth is teeming with life, due in part to its orbit in a narrow zone. Any closer to or further from the Sun and water would boil away or freeze solid. Between those boundaries lies a region called the habitable zone. When looking for signs of life in an otherwise cold and dead universe, that is the place to look.
While to us it seems large and significant, our sun is what is considered to be an average yellow dwarf. For comparison, Betelgeuse is a red giant about 880 times larger than the Sun. If it were placed in the center of the solar system, Betelgeuse would extend past Earth’s orbit. On the other end, many stars are very small, more comparable to the size of Jupiter. Jupiter is a gas giant that is mainly hydrogen, and at its core, there is such intense heat and pressure that hydrogen begins to act like a metal. However, it is not yet massive enough to fuse two hydrogen atoms together and ignite into a star. A body that is 50-100 times more massive than Jupiter can fuse hydrogen, but is still not very large, nor is it particularly luminous. Those common stars are referred to as red dwarfs. The closest star to us, Proxima Centauri, at 4.2 lightyears, is a red dwarf, recently found to be orbited by a rocky planet in its habitable zone.
A size comparison of TRAPPIST-1 to Jupiter, its moons, the terrestrial planets, and the Sun.
In late 2015, the Transiting Planets and Planetesimals Small Telescope in Chile made its first discovery: three planets orbiting a red dwarf star with the absolute mess of a name, 2MASS J23062928-0502285. They promptly renamed it TRAPPIST-1. (To save space, I’ll refer to it as T-1.) Subsequent observations from the Spitzer Space Telescope have more than doubled the known planets around T-1 to seven. Of these, three are confirmed to be well within the habitable zone of the star. They are also Earth-sized and mainly rock. While sufficient atmospheric data hasn’t been gathered, once the James Webb Space Telescope is operational late next year, we should be able to confirm if the air (if any) is breathable or if it contains signs of life.
Unfortunately, air isn’t the only factor in habitability. Stars are not a fun things to be near. A phenomenon known as coronal mass ejection happens frequently for our star, multiple times a day during solar maximums. A CME is when plasma gets caught in a twisting of the magnetic field and erupts from the outermost region of the star’s atmosphere, sending charged particles out into the solar system. If not for Earth’s magnetic field, the particles could strip away our atmosphere (though impacts do still happen) and thus its ability to sustain life. Mars lacks a magnetosphere and has lost much of its atmosphere, a key factor in it being barren. CMEs often come from starspots, which are regions where the tangles magnetic field of a star poke through the surface, dimming the region. The dimming is from an effect of quantum physics, but the result can reduce the light received by a planet by 40 percent, cooling it significantly. Conversely, some dwarfs are flare stars, which can dramatically increase in brightness in a matter of minutes, which in turn raises the temperature of the planet.
Earth orbits at a distance of roughly 93 million miles, which is often called one astronomical unit, or one AU. The closest planet to T-1 orbits at .011 AU. One “year” on this planet takes 36 hours. T-1’s furthest known planet orbits at .063 AU with an orbit of 20 days.
Close orbits to such a massive object are believed to tidally lock the planets, meaning one side is always facing the star and the other is always facing away (The Moon is tidally locked to Earth.) Life on those planets would thus be constrained to the narrow region between hemispheres where temperatures would be moderate. That’s assuming there isn’t a significant atmosphere to distribute the dayside heat throughout. But then, that atmosphere is an assumption itself. A tidally locked body would rotate once per orbit and it would be unlikely that its molten core would rotate fast enough to produce a magnetic field strong enough to deflect the solar particles vying to strip the atmosphere away. Given the close proximity to the star, an abundance of particles is almost a given.
The TRAPPIST-1 system, for all the complications, can still harbor life. Life evolved on Earth through unlikely circumstances, and it is the only planet in our system with liquid water. Three of the newly discovered planets have the right conditions for water, and where there’s water, life seems to follow. It is thought that the system is still young (if you can consider upwards of 500 million years young), so life there still has a long way to go. Whatever life it may be, it is unlikely it will be remotely close to life as we know it. But we have to find it first.
Further reading (and some nice graphics) can be found at http://www.trappist.one/
@ThinkinAbtSpace
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