No less than seven Earth-sized exoplanets have been discovered orbiting Trappist-1, a tiny star in the constellation of Aquarius, 40 light years from the Earth. Three of the planets seem to be within the sweet spot known as the habitable zone, which means that all other things being equal, they could support life, scientists announced yesterday.
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"It is the first time that so many planets of this kind have been found around a single star," Michael Gillon of the Universite de Liege, Belgium told reporters. "They form a very complex system, very close to the star, reminiscent of the system of moons around Jupiter."
Trappist-1 being a dwarf star, about 8% of the sun's mass, was crucial to the team's choice to look at it. Other teams seeking exoplanets tend to focus on bigger celestial bodies.
"The star being much smaller – the planets cast a relatively much larger shadow," explains Amaury Triaud of Cambridge.
Actually, the team, Gillon and Jehin Emmanuel of the Universite de Liege and Triaud identified the first three planets orbiting Trappist-1 a year ago, based on core-accretion theory's prediction that given the small masses of these stars, and the small sizes of their protoplanetary disks, they should have terrestrial planets orbiting them. Having found three, they trained more attention on the star and found not three but seven exoplanets, which just means any planet not orbiting our Sun.
Some of the seven are a bit smaller than Earth, some a bit larger, by about 10% to 20%, the team says.
Of the seven exoplanets, they report now, six seem to be in the sweet spot temperature-wise, between zero and 100 degrees. (Though at the upper end of that range, the only Earth-bound life a planet could support is thermophilic archaebacteria.) The tolerable temperature ranges are because In contrast to our hot sun, Trappist-1 is an ultracool dwarf star, which means that its effective temperature is less than 2,700 kelvin, equivalent to 2,430 degrees Celsius or 4,400 degrees Fahrenheit.
The inner two planets receive four times and two times the irradiation of Earth, respectively, Gillon and his team reported last year.
The star Trappist-1's real name is 2MASS J23062928-0502285. Its exoplanets have been designated Trappist-1A, 1B and so on. The "trappist" comes from Transiting Planets and Planetesimals Small Telescope.
Tiny little eclipse far, far away
The planets were detected using transit photometry, which is a technique to detect when a planet passes between us and its parent star, dimming the starlight we see as it transits. The light doesn't dim much, but the observed visual brightness drops enough for us to detect, with serendipity and clever telescopes.
The degree of dimming depends on the relative sizes of the star and the planet, though size alone cannot tell us whether the planet is a primarily gaseous type like Jupiter, or a rocky type like Earth.
The team estimates that the masses of the six inner planets are similar to Earth's. What their atmospheres are like is anybody's guess at this point, though advanced study of the two innermost planets - atmosphere, size and mass – indicate they do not have big thick envelopes and are primarily rocky.
That's all we can say for now. "These planets could have some liquid water and maybe life on their surface," Triaud speculates, though elucidating that could take another decade, he adds.
Directly detecting life on exoplanets is impossible at this stage of technology. We can potentially detect telltale biological chemicals in their atmospheric composition. As Gillan points out, there are quite a lot of possible molecules that could attest to biological activity. In 2008, using the Hubble telescope and spectroscopy, scientists made the first detection of an organic molecule in an exoplanet's atmosphere – methane, in the sky of HD 189733b.
On the upside for potential life, Trappist-1 is a very quiet star, say the scientists: It has some flares but not particularly powerful ones. Also, Triaud elaborates, "The energy output from dwarf stars like Trappist-1 is much weaker than that of our Sun. Planets would need to be in far closer orbits than we see in the Solar System if there is to be surface water. Fortunately, it seems that this kind of compact configuration is just what we see around Trappist-1."
Intriguingly, while cosmologists believe the solar system planets by and large formed around the Sun from an accretion disk of cosmic crud, the MIT-Liege team postulates that Trappist-1's planetary companions came from the outside, Gillon says.
So far almost 2,000 exoplanets have been found, several around other stars in Aquarius.
While we continue to ponder whether we are alone, we are by now confident that exoplanets are not rare. Whether any host life and whether the life would like us, be hostile to us as Stephen Hawking suspects, or be supremely indifferent, is another question.