View Full Version : Good News For The Search For Life, The Trappist System Might Be Rich In Water

2018-Feb-05, 10:20 PM
Scientists have been hard at work trying to determine the densities of the TRAPPIST-1 planets, and it looks like water is abundant in the TRAPPIST system.
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2018-Feb-11, 03:27 PM
Quick intros:
Not So Strange New Worlds - NASA Spitzer Space Telescope (http://www.spitzer.caltech.edu/video-audio/1685-ssc2018-04v1)
Imagining the Planets of TRAPPIST-1 - NASA Spitzer Space Telescope (http://www.spitzer.caltech.edu/video-audio/1690-ssc2018-04v2)

Very technical:
[1703.01424] Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1 (https://arxiv.org/abs/1703.01424) (early 2017)
[1704.04290] Updated Masses for the TRAPPIST-1 Planets (https://arxiv.org/abs/1704.04290) (early 2017)
[1802.01377] The nature of the TRAPPIST-1 exoplanets (https://arxiv.org/abs/1802.01377) (the most recent one)

The TRAPPIST-1 planets are observed with their transits, and that directly gives only their sizes and orbits. However, one can get the amount of atmospheric extinction (absorption + scattering) at different wavelengths by finding planets' effective sizes at those wavelengths.

But some exoplanets are in orbital resonances with some other ones, and that amplifies the effects of their gravitational pulls on each other. This effect produces observable Transit Timing Variations (TTV's) in several exoplanets, including the TRAPPIST-1 ones. This means that we have both masses and radii for all seven of these planets.

All seven have radii, masses, densities, and surface gravities rather close to our planet's.

Masses and radii are only enough to determine planets' compositions if the planets are made of only two materials. But we can plausibly expect at least three: iron, rock, and water, meaning that a planet might be mostly rock, or else some mix of iron, rock, and water, while having the same mass and density. So I used the paper's estimate of the iron/rock ratio, a little bit less than for the Solar System. I find these relative masses of water:

b: 0.05, c: 0.02, d: 0.05, e: ~0, f: 0.02, g: 0.04, h: 0.03

Error bars: ~ 0.1

Those numbers don't look like much, but the Earth has 0.00023 for its oceans. Those oceans' average depth is 3.7 km, and averaged over all the planet's surface, 2.6 km. So I find these estimated ocean depths:

b: 400, c: 200, d: 250, e: ~0, f: 250, g: 400, h: 150 km

Error bars: ~ 100 km

So at least 6 of the 7 planets have superdeep oceans by Earth standards.

But in the case of as much rock as possible, then only b, d, and g have thick oceans (250, 150, 250 km), with f being borderline.