The Seven Worlds of TRAPPIST-1

Imagine you are visiting a distant planet where daylight has a salmon hue but the intensity is much less than what you would normally encounter on the Earth. You are warm nevertheless because the planet belongs to a red dwarf star which looms big in the sky, almost three times as big as our Sun. You watch other planets crisscrossing the sky in the ruddy, perpetual twilight, some of them similar to the moon in our Earthly sky and some even bigger! Behind you, gleaming in the star’s reddish glow, lies the deep ocean that covers a large part of this mystic planet.

Interestingly, this is how the world would hypothetically yet plausibly appear in a planet called TRAPPIST-1f — a member of a seven-planet system belonging to a star named TRAPPIST-1 — located about 40 light years away from us, in the constellation Aquarius. TRAPPIST-1 is a red dwarf star whose mass is just eight per cent that of the Sun. Red dwarfs, also called “M-dwarfs”, are the most abundant stars in the galaxy. They are too faint to be spotted with the naked eye but their reduced size and limited radiance have an important consequence. They are the longest-lived stars of the galaxy with lifetimes far greater than our Sun. The reason they are so long-lived is because they consume their supply of hydrogen fuel at a much slower pace than more massive stars such as our Sun.

In May 2016, an international team of researchers using TRAPPIST (TRAnsiting Planets and Planetesimals Small Telescope) in Chile reported discovery of three Earth-sized planets orbiting this Jupiter-sized star. An ensuing detailed photometric monitoring of the star (that is, astronomers measured the dipping of the star’s light when each planet passed or transited in front of its face), with the global network of ground-based TRAPPIST and space-based Spitzer telescopes, revealed four more planets orbiting this star — a total of seven — whose sizes and masses are similar to the Earth. Based on their densities, all of the TRAPPIST-1 planets — dubbed TRAPPIST-1b through h (Figure 1) — are thought to be rocky. They huddle very close to their host star in tightly separated orbits. The orbital periods of the six inner planets are only 1.51, 2.42, 4.04, 6.06, 9.1 and 12.35 days while that of the outermost planet is uncorroborated. Computer models suggests that the planets could have formed farther from the star and migrated inwards at some points in their cosmic histories.

Figure 1: Seven Earth-sized planets around a tiny, cold red dwarf star called TRAPPIST-1. Three of these planets TRAPPIST-1e, f, and g, are firmly in the habitable zone. Credits: NASA

The discovery was deemed very newsworthy because of its implications in the ongoing search for extraterrestrial life and NASA decided to announce the findings at a news briefing at their Headquarters in Washington. The results appeared in the journal Nature on May 12, 2016.

Can the seven worlds of TRAPPIST-1 support a habitable ecosystem system? It is too early to tell but there is a chance that some of them could. The innermost planets, TRAPPIST-1b, c and d, are probably too hot to support liquid water, although a small amount could exist somewhere on their surfaces, while TRAPPIST-1h is too distant and cold to harbor liquid water. TRAPPIST-1e, f, and g, are the most interesting planets as their orbits are confined within the habitable zone — the region where oceans of liquid water could exist on their surfaces, and thus life would be considered most likely to evolve.

Planets of red dwarf stars were once considered unfit for life to thrive because the habitable zones of red dwarfs overlap with their radiation belts where massive flares of X-rays and UV radiation are commonplace. For a dim star like TRAPPIST-1, which radiates much less heat than the Sun, the habitable zone lies much closer to the star. But the flares are the death knell of life as we know it. Besides, some of the closest planets could be tidally locked — the side constantly facing the parent star turning extremely hot, while the far side, extremely cold. But new computer models show that some planets in this configuration could still harbor life provided their atmospheres could dissipate heat across the surfaces. This is a significant result as the chances of evolution of life in the universe is greatly bolstered because red dwarfs make up more than three-quarters of the stars in the galaxy.

Curiously, discovery of the seven worlds of TRAPPIST-1 in some way has vindicated the long-standing imagination of many science-fiction authors who have depicted solar systems comprising of “space-faring civilizations” with multiple Earth-like planets conveniently suitable for human settlement. However, there is no guarantee that these planets could support life. Yet, how exciting will it be if we eventually find out a couple of them really do? Paying a visit is out of question with the current rocket technology because they are too far from us. The idea is to scientifically study these planets by remotely observing them with powerful telescopes. This is precisely the goal of the next phase of research: study the atmospheric composition of each planet, determine whether liquid water truly exists on their surfaces and search for possible signs of life-supporting atmospheres using the Hubble Space Telescope. Hubble’s successor, the James Webb Space Telescope, scheduled to be launched in 2018, will be ideally suited to provide a closer look at their atmospheric chemistry. If these planets indeed have atmospheres, they really are the best places to look for life.

Astrophysicist and author Ethan Siegel has beautifully summarized the excitement thus: “While the discovery of these seven planets is remarkable, the most interesting part of this story is yet to be written. As our telescopes become larger and more sophisticated, we’ll finally gain the ability to measure the spectra of these worlds’ atmospheres, search for signs of water and life, and perhaps even discover an answer to whether we’re not alone in the Universe as far as life goes. With three strong candidates, we might finally need to face the possibility that most of the life that arises in the Universe might occur around stars that aren’t like the Sun!”

Interestingly, the TRAPPIST-1 Solar System will probably long outlive ours because M-dwarf stars are extremely long-lived. When our Sun eventually dies, TRAPPIST-1 will still be a relatively young star and may go on to shine for another trillion years. If there is another part of the universe destined for life to persist, it could very well be in the Solar System of TRAPPIST-1, a red dwarf star located in the constellation Aquarius, 40 light years away.

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