TRAPPIST-1 new results: more precise masses

[lpetrich]

I'd discussed TRAPPIST-1 and its seven planets in some posts in Only one planet per star? - Secular Café, but some new results have come out about those celestial bodies, and I decided to headline them.

Quick intros:
Not So Strange New Worlds - NASA Spitzer Space Telescope
Imagining the Planets of TRAPPIST-1 - NASA Spitzer Space Telescope

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

The TRAPPIST-1 planets are observed by the transit method, by watching them cross across their star and block some of their star's light. That method gives their sizes, but not much else about them. However, the planets are in orbital resonances, and that amplifies their perturbations of each others' motions. This produces observable Transit Timing Variations (TTV's), and this effect has been used to determine the masses of several exoplanets, including the TRAPPIST-1 ones.

Here's a table of the planets' masses in Earth masses and how they've been improved from paper to paper:
Planet | 17-1 | 17-2 | 18 | Radius (RE) b | 0.85 ± 0.72 | 0.79 ± 0.27 | 1.017 + 0.154 - 0.143 | 1.121 + 0.031 - 0.032 c | 1.38 ± 0.61 | 1.63 ± 0.63 | 1.156 + 0.142 - 0.131 | 1.095 + 0.030 - 0.031 d | 0.41 ± 0.27 | 0.33 ± 0.15 | 0.297 + 0.039 - 0.035 | 0.784 + 0.023 - 0.023 e | 0.62 ± 0.58 | 0.24 + 0.56 - 0.24 | 0.772 + 0.079 - 0.075 | 0.910 + 0.026 - 0.027 f | 0.68 ± 0.18 | 0.36 ± 0.12 | 0.934 + 0.080 - 0.078 | 1.046 + 0.029 - 0.030 g | 1.34 ± 0.88 | 0.566 ± 0.038 | 1.148 + 0.098 - 0.095 | 1.148 + 0.032 - 0.033 h | -- | 0.086 ± 0.084 | 0.331 + 0.056 - 0.049 | 0.773 + 0.026 - 0.027

As you can see, the error bars are much improved over the earlier calculations, and we now have good masses for all these planets. Combined with their radii, this gets their average densities:

b: 4.00, c: 4.87, d: 3.40, e: 5.64, f: 4.50, g: 4.18, h: 3.96

That's 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 in km:

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

Error bars: ~ 100

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

[Aupmanyav]

Nice info. They might have more/bigger fish.

[Jobar]

Perhaps the matter of how much water a terrestrial planet has should be included as one of the terms of the Drake equation. Certainly if a planet has no land surface, it's most unlikely that a technological race could evolve there.

If there's any tectonic activity under the water, though, I should think life might form. But I have no idea if photosynthesis would evolve, or an oxygen atmosphere.

[lpetrich]

[quote=""Jobar""]Perhaps the matter of how much water a terrestrial planet has should be included as one of the terms of the Drake equation. Certainly if a planet has no land surface, it's most unlikely that a technological race could evolve there.
[/quote]
Or else it would be a factor in ne.

It is remarkable how far we have come with exoplanets, even though our information on them continues to be very limited.

If there's any tectonic activity under the water, though, I should think life might form. But I have no idea if photosynthesis would evolve, or an oxygen atmosphere.

It would be a long way up from the ocean floor, and there is a serious prospect of nutrient starvation. Desert Dust Feeds Deep Ocean Life - Scientific American -- without eroding landmasses, the oceans may have much less of trace-element nutrients like iron. Hydrothermal vents will supply some such nutrients, but I'm not sure how much.