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Planetary science

TRAPPIST-1 exoplanets could harbour significant amounts of water

13 Feb 2018
Artist’s impression of the TRAPPIST-1 exoplanets
Habitable worlds: artist’s impression of the TRAPPIST-1 exoplanets

The chances that several exoplanets in the TRAPPIST-1 system could be habitable have been boosted by new measurements that push the envelope of what exoplanet science can do with today’s telescopes.

The TRAPPIST-1 system is just 39.6 light-years away and comprises seven small worlds that orbit a lone red dwarf star. The inner three worlds were discovered in 2016 by astronomers using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) at the European Southern Observatory in Chile. The outer four planets were spotted a year later and it is it is possible that all seven worlds could potentially be habitable.

Now, new results have narrowed down the masses of the planets, confirming that they are all likely to be rocky, without the extended atmospheres that miniature versions of Uranus and Neptune would have. Furthermore, five of the planets have densities that suggest a significant amount of water, which is vital for life as we know it.

Transit timing variations

Astronomers led by Simon Grimm of the Centre for Space and Habitability at the University of Bern in Switzerland have produced the most accurate calculations of the planetary masses yet by taking advantage of a phenomenon known as transit timing variations, or TTVs. The seven worlds gravitationally push and pull on each other, resulting in the timing of their transits in front of their star being delayed or advanced by up to 1 h.

In the TRAPPIST-1 system, “the TTV are much stronger and more complicated than in many other systems that have fewer exoplanets,” says Grimm. The challenge of disentangling the data to calculate the planetary masses required new code written to handle 35 different parameters, five for each exoplanet. These are mass; orbital period; eccentricity; the argument of perihelion (the angle between its perihelion position and where the exoplanet’s inclined orbit passes through the ecliptic plane); and the mean anomaly (an angle required to calculate an exoplanet’s location on its elliptical orbit at any given time). The code produced a range of different solutions, from which Grimm’s team determined the configuration that best fits the observational data.

Water worlds

The most massive of the seven worlds is exoplanet “c”, the second from the star, with a mass 1.156 times that of Earth. The least massive is exoplanet d, with less than a third of Earth’s mass. The magnitude of the transits tells astronomers the radii of the exoplanets, and from their radius and mass, their densities can be calculated.

This is where things start to become interesting. Based on their densities, all seven worlds are predominantly rocky, but contain up to 5% water. This is much more water than Earth’s oceans (which amount to just 0.02% of Earth’s mass). However, it remains to be seen whether the TRAPPIST-1 water is present on the surface of the exoplanet in vast, deep oceans, or whether it is as vapour in a dense, steamy atmosphere, or whether it is spread around inside the exoplanet, much like how Earth’s mantle contains the equivalent amount of water as in the oceans.

Based on its temperature, exoplanet e would be the most similar to Earth according to Grimm. This world has 77% of the mass of Earth, but is a little denser, indicating a large iron core and a thin atmosphere, possibly even thinner than Earth’s.

Hubble finds no hydrogen

Meanwhile, new observations by the Hubble Space Telescope support the conclusions of the TTV calculations, confirming the likely terrestrial status of the TRAPPIST-1 exoplanets by ruling out the existence of the extended hydrogen envelopes found in the atmospheres of Uranus and Neptune. Pushing Hubble’s powers of resolution to the limit, a team led by Julien de Wit of the Massachusetts Institute of Technology using Hubble’s Wide Field Camera 3 to make infrared spectroscopic observations did not detect any large, puffy atmospheres around exoplanets d, e, f and g, “leaving many more terrestrial-like possibilities to be explored with future telescopes such as the James Webb Space Telescope,” says Hannah Wakeford, a member of de Wit’s team from the Space Telescope Science Institute in Baltimore.

Hubble had previously searched for hydrogen-rich atmospheres around the innermost exoplanets b and c in 2016, while observations of the outermost world, h, remain inconclusive. The next step is to look in the ultraviolet for hydrogen escaping from the exoplanets’ atmospheres. For the innermost worlds, this would be a sign of a greenhouse effect, whereby the temperature boils the oceans, filling the atmosphere with water vapour, which is then broken down into oxygen and hydrogen by ultraviolet light from the star, allowing the hydrogen to escape into space. It is the same scenario that has taken place on Venus.

Best opportunity

However, Wakeford says that while it is difficult to draw any direct analogues to the Sun’s planets, TRAPPIST-1 “still represents the best opportunity we have for studying Earth-sized worlds outside of our own solar system”.

The research is described in Nature Astronomy and in an upcoming paper in Astronomy and Astrophysics.

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