Frontiers Physics World  March 2018

Boost for chance of exoplanet life

Potential for life Artist’s impression of the TRAPPIST-1 exoplanets. (NASA / JPL-Caltech)

The possibility that some of the exoplanets in the TRAPPIST-1 system could harbour life has been boosted by new measurements. The findings, which narrow down the possible masses of these seven Earth-like planets, yield densities that indicate all are likely to be rocky. Five of the planets also have as much as 5% water, which is vital for life as we know it.

Located just 40 light years from Earth, the TRAPPIST-1 planets orbit an ultracool, red-dwarf star. The inner three planets were discovered in 2016 using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, while the outer four planets were spotted a year later. Although researchers had determined the sizes of the planets to better than 5%, there were big uncertainties in their mass and density, which is vital for deducing their structure.

The new measurements were taken by an international team of astronomers led by Simon Grimm from the University of Bern in Switzerland. They calculated the masses of the TRAPPIST-1 planets by monitoring small variations in the times at which each passes in front of the parent star. These “timing transit variations” (TTVs), which are caused by the seven planets gravitationally pushing and pulling on one another, can delay or advance those times by as much as one hour.

The TTVs in the TRAPPIST-1 system are stronger and more complicated than in many other systems with fewer exoplanets, forcing Grimm’s team to write complex software code to calculate the planetary masses. It revealed that the most massive of the seven worlds is exoplanet c, the second from the star, with a mass 1.156 times that of Earth, while the lightest is d, weighing less than a third of our planet.

The resulting densities – calculated by knowing the radii of the exoplanets from the size of the transits – indicated that planets b, d, f, g and h are covered in volatile gases with up to 5% water. It is unclear, however, if the water is in vast oceans, vapourized in the atmosphere or spread inside the planet (A & A 10.1051/0004-6361/201732233).

Separate infrared spectroscopic measurements made by the Hubble Space Telescope’s Wide Field Camera 3 indicated that planets d, e and f are unlikely to be rich in hydrogen. This finding supports the idea that they could harbour life because hydrogen is a powerful greenhouse gas that would prevent life on inner planets (Nature Astron. 10.1038/s41550-017-0374-z).

Keith Cooper