Fresh evidence of life on TRAPPIST-1 planets

A series of four studies have shed new light on the properties of the TRAPPIST-1 planetary system, currently our most optimal hope for evidence of biological life beyond the Solar system.Since the extent of the TRAPPIST-1 planetary system was revealed in February 2017, it has captured the imagination of people the world over.

The new studies, including papers published Monday in Nature Astronomy and Astronomy and Astrophysics, are the result of researchers working to better characterise the planets and collect more information about them.The international team first refined the properties of the star at the centre of the system, and secondly improved the measurements of the planets’ radii. A third study offers better estimates than ever for the planets’ masses while in the fourth study the team performed reconnaissance observations of the planets’ atmospheres.

The four international studies were produced in collaboration with University of Birmingham, United Kingdom (U.K.), astronomer, Dr. Amaury Triaud. He explains, “After discovering this incredible planetary system our team was extremely eager to know more about TRAPPIST-1. A year on, we are reporting our results. Thanks to our efforts the TRAPPIST-1 planets are becoming the best studied worlds outside the Solar system.”The team found that all seven planets are mostly made of rock, with up to five percent of their mass in water – a significant amount. By comparison, our Earth’s oceans account for only 0.02 percent of our planet’s mass.

In addition, five of the planets appear devoid of an atmosphere made of Hydrogen and Helium, like for Neptune or Uranus. This new information reinforces the notion that the seven planets of TRAPPIST-1 are similar to the rocky worlds of the Solar system in many ways.The form that water takes on TRAPPIST-1 planets would depend on the amount of heat they receive from their star, which is a mere nine percent as heavy as our Sun.

The seven planets are considered temperate, meaning that under certain geological and atmospheric conditions, all could possess conditions allowing water to remain liquid. Work, including the team’s series of results, is now proceeding to pinpoint which of these temperate planets are most likely to be habitable.
Of the seven, TRAPPIST-1e, the fourth from the star, is currently the most akin to Earth although much remains to be known, notably the conditions at the surface, and whether it holds an atmosphere.

TRAPPIST-1 is named for the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, which discovered two of the seven planets we know of today — announced in 2016. NASA’s Spitzer Space Telescope, in collaboration with ground-based telescopes, confirmed these planets and uncovered the other five in the system. Since then, NASA’s Kepler space telescope also observed the TRAPPIST-1 system, and Spitzer collected additional data. This new body of data helped the team paint a clearer picture of the system than ever before — although there is still much more to learn about TRAPPIST-1.

The TRAPPIST-1 planets huddle so close to one another that a person standing on the surface of one of these worlds would have a spectacular view of the neighbouring planets in the sky, which would sometimes appear larger than the Moon looks to an observer on Earth. They may also be tidally locked, meaning the same side of the planet is always facing the star, and each side is in perpetual day or night. Although the planets are all closer to their star than Mercury is to the Sun, TRAPPIST-1 is such a cool star that its planets are temperate.

Also, an international team of astronomers has used the NASA/ESA Hubble Space Telescope to look for atmospheres around four Earth-sized planets orbiting within or near TRAPPIST-1’s habitable zone. The new results further support the terrestrial and potentially habitable nature of three of the studied planets. The results are published in Nature Astronomy. Seven Earth-sized planets orbit the ultracool dwarf star TRAPPIST-1, 40 light-years away from the Earth. This makes TRAPPIST-1 the planetary system with the largest number of Earth-sized planets discovered so far. These planets are also relatively temperate, making them a tantalizing place to search for signs of life beyond our Solar System. Now, an international team of astronomers has presented a study in which they used the NASA/ESA Hubble Space Telescope to screen four planets in the system — TRAPPIST-1d, e, f and g — for traces of atmospheres.

Three of the planets are within the system’s habitable zone, the region at a distance from the star where liquid water — the key to life as we know it — could exist on the surface. The fourth planet orbits in a borderline region at the inner edge of the habitable zone. The data obtained rule out a cloud-free hydrogen-rich atmosphere for three of the planets — but for the fourth planet, TRAPPIST-1g, such an atmosphere could not be excluded.