The Worlds of TRAPPIST-1

The internet is alight with people sharing the news of NASA's latest announcement. After teasing us with talk of a discovery "beyond the solar system," NASA announced the existence of a seven-planet star system located less than forty light years away. What's most exciting about the discovery is that most of the planets are considered potential habitats for life.

The star known as TRAPPIST-1 is, like many modern stellar discoveries, named after the device used to detect it, in this case, the TRAnsiting Planets and Planetesimals Small Telescope in Chile. You may notice that this doesn't quite spell out TRAPPIST, but the inventors are obviously Belgian beer fans and I can support that. Located 39 light years away in Aquarius, TRAPPIST-1 is described as an ultra-cool dwarf star, on the borderline of a red dwarf and a sub-stellar brown dwarf. Three planets were discovered there and announced back in 2015, and since then, the NASA Spitzer telescope and the Very Large Telescope at Paranal have detected four more. The extent of the system was announced last night (Feb 22nd).

In spite of being such a cool, dim star, TRAPPIST-1 may make an effective sun for life-bearing planets due to their extremely close orbits. All seven known planets orbit the star closer than Mercury orbits the Sun, and at least three of them are considered right within the system's habitable zone. This makes the planets astonishingly close together; the first and second planets are only slightly further apart than the Earth and the Moon. Finally, elaborate skies hanging with enormous sister planets can be considered a reality, not just science fiction. 

OK, this is Vulcan, but you get the idea

The planets are named TRAPPIST-1b through to -1h, and, unusually for discoveries of this type, are labelled in order of distance from their star. Each of them is within Earthlike mass and radius, with at least three of them estimated as being smaller than the Earth, and all are considered to be rocky, terrestrial-type planets. TRAPPIST-1b and -1c are the closest, with featureless spectra that indicates either a cloudless, water vapour dominated atmosphere, or a thicker, Venus-type atmosphere. They most likely lost the bulk of their surface water while their star was still cooling, and are less likely as abodes for life. TRAPPIST-1d is more likely habitable, although still closer than the calculated Goldilocks zone. TRAPPIST-1e, -1f and -1g are right within this zone, and are probably fairly cool in comparison to their inner brethren, far more likely to hold liquid water, Depending on the thickness of the atmosphere, they may be cooler than the Earth, or more comfortably terrestrial. TRAPPIST-1h is less well analysed so far, but is likely cold and less hospitable.

While the relative positions of planets to the star suggest potentially life-supporting temperatures, we shouldn't jump to conclusions. As always, Earthlike is a relative term. Red dwarf stars, let alone ultra-cool dwarfs, are debatable as life-supporting stars, due to the extreme proximity of their planets. The year on these planets will be very rapid, in the manner of a few days (-1b's year lasts only a day-and-a-half in Earth terms, with -1h at no more than 35 days, probably less). They are also likely to be tidally locked, with one side of each planet permanently facing the star. Both facts would lead to extreme weather conditions on the surface. Equally, radiation from the star, including X-rays and extreme energy ultraviolet radiation, would bombard the planets constantly at that proximity, depleting the atmospheres and making it harder for life to form.

Still, there is reason to hope. Even if the planets are lifeless now, they may not be always. Stars' longevity is inversely proportional to their size and temperature, and an ultra-cool dwarf like TRAPPIST-1 is likely to last a thousand times as long as the Sun, remaining stable for trillions of years. TRAPPIST-1 is estimated at only 500 million years old at present, but it could become a host for life for many thousands of millions of years in the future. 

Click the link here for NASA's announcement and an artist's impression of planet TRAPPIST-1d.