Originally posted to 13.7 Cosmos & Culture on 2/23/2016
A group of astronomers announced last week that seven Earth-size planets orbit a small, red, dwarf star 40 light-years away.
The findings were published in the journal Nature. Observations indicate that at least three of the planets may be at temperate zones where liquid water may exist.
The extraordinary finding — discovered by astronomers from an international collaboration led by Michaël Gillon from the University of Liège in Belgium, places the search for Earth-like planets and, more spectacularly, the search for alien life, under a brand-new lens. NASA released a fun poster about the findings.
The small star, about 12 times smaller than our sun (think basketball to ping-pong ball), and with a temperature of only 4,150 degrees Fahrenheit on its surface (our sun's is about 10,000 degrees), is named Trappist-1, not for the strict order of Roman Catholic monks, but short for Transiting Planets and Planetesimals Small Telescope. "Transit" here refers to a specific technique astronomers use to detect planets orbiting distant stars. A planet passing in front of the star dims its light slightly. Tracking the amount of dimming and its periodicity, astronomers can decode how far the planets are from the star, their approximate sizes, and how many there are as well. Of course, the closer and larger the planets, the more of an impact they make and the easier they are to spot.
In the case of Trappist-1, Gillon and collaborators said they suspected at least three orbiting planets in a paper they published last year. Since then, more ground-based and orbiting telescopes observed the star, including NASA's infrared Spitzer Space Telescope. With better data, the number of planets climbed quickly. The seven spotted are very close to the star, the first six with orbits varying from 1.5 days to 12.5 days. (There's still some uncertainty regarding the seventh planet's orbit, but it seems to be about 20 days long.) For comparison, Mercury takes 88 days to orbit the sun.
So far, astronomers confirmed that the first two planets are rocky (that is, not made mostly of hydrogen, as are our own gas giants, Jupiter and Saturn). The fourth, fifth, and sixth planets orbit within the star's "habitable zone," a belt around the star where conditions allow for liquid water. This is the first indication astronomers use to search for alien life. In our solar system, Venus, Earth and Mars are in the sun's habitable zone. Clearly, habitable zones must be taken with a grain of salt, given that local planetary conditions, such as atmospheric composition or active volcanism, may dramatically affect habitability. Also, given how close the seven planets are to their star, they are probably in tidally-locked orbits, always showing the same face to their star, as our moon does to us.
A visitor would probably see a dim-lit sky under a salmon-colored star. Even if tidally-locked, atmospheric dynamics would circulate the heat around the dark side of the planets, allowing for potential life to develop in different niches. As we know from our own biosphere, life thrives at the boundaries between different habitats, such as ocean shallows or river banks.
The next step is to identify the dominant elements in the planets' atmospheres. Due to the relative proximity of this remarkable planetary system, ground-based telescopes and the Hubble Space Telescope will likely be able to discern some them. If water, oxygen, ozone, methane and carbon dioxide are found within certain proportions, chances of some kind of life (as we know it) are very high. Finding life in the Trappist-1 system would mean that, given the right ingredients, life is likely to emerge.
If it's here and there, it's probably in many other spots across the galaxy, especially considering that cool, red, dwarfs are the most common stars. The conclusion would be a bombshell: life ubiquitous in the universe. If, on the other hand, life's atmospheric signatures are not found in the Trappist-1 system, the conclusion would also be a bombshell: Life is probably rarer than many anticipate.
So stay tuned, as we may have an answer within five years or so.