Three new planets discovered around a tiny, faint star in the constellation Aquarius may be among the best places to look for evidence of extraterrestrial life. A team of astronomers led by Michaël Gillon of the Institute of Astrophysics and Geophysics at the University of Liege in Belgium studied the star, named TRAPPIST-1, and discovered it faded in regular cycles. Analysis of the repeated dips in brightness revealed that three planets all about the size of the Earth orbiting the star were responsible.
TRAPPIST-1 is an ultra-cool red dwarf star only a little larger than Jupiter located 40 light years from Earth. Red dwarfs are far less massive than the sun and are probably the most common type of star in the galaxy. They burn their nuclear fuel so frugally, dwarfs have long lifetimes measured in the trillions of years compared to the 10-billions lifetimes allotted to larger stars like our sun that devour their fuel more quickly. This is the first time that planets have been found around one of them.
Emmanuël Jehin, a co-author of the new study, is excited: “This really is a paradigm shift with regards to the planet population and the path towards finding life in the universe. So far, the existence of such ‘red worlds’ orbiting ultra-cool dwarf stars was purely theoretical, but now we have not just one lonely planet around such a faint red star but a complete system of three planets!”
Nearly all of the 2,111 extrasolar planets discovered to date orbit brighter stars. Astronomers search for signs of life by looking for fingerprints of life-related gases like oxygen, methane and water vapor when light from the host star passes through the atmosphere of a transiting planet. For Earth-sized planets orbiting most stars this tiny effect is swamped by the brilliance of the starlight. Only for the case of faint red ultra-cool dwarf stars like TRAPPIST-1 is this effect big enough to be detected.
Follow-up observations with larger telescopes, including the European Southern Observatory’s (ESO) 8-meter (315-inch) Very Large Telescope in Chile, have shown that the planets orbiting the dwarf have sizes very similar to that of Earth. Two of the planets orbit the star about every 1.5 days and 2.4 days respectively, and the third planet somewhere in the range of 4.5 to 73 days.
Those short orbital periods indicate that the planets are between 20 and 100 times closer to their star than the Earth to the sun. Despite that proximity, the inner two planets only receive four time and twice the amount of radiation received by the Earth because TRAPPIST-1 is so much fainter than the sun. The same way you or I might huddle closer to a small fire to soak up its heat, the three planets’ close orbits ensures they feel their star’s warmth.
They might even be a little too close; astronomers place them just outside the “habitable zone”, the region around a star where a rocky planet with a thick enough atmosphere can support liquid water on its surface. That’s doesn’t rule out regions on their surfaces that might be clement, but only time and further observations will tell. Scale-wise, Gillon compares the system to Jupiter and its moons.
We shouldn’t have to wait long to find out if any of the three possesses water or possible traces of biological activity. The European Extremely Large Telescope, a 39-meter (1,535-inch) expected to see first light in 2024, and the James Webb Space Telescope, set to launch in 2018, will have the firepower to study their atmospheric compositions.
Since about 15% of the stars near the sun are ultra-cool dwarfs — I can’t shake the image of stars wearing sunglasses — we’re only beginning to knock on the doors of Earth’s cousins.