TOI-178

We have astronomy news from 200 years ago at TOI-178. It's a K star, of type sometimes marked as prime for habitability. Earlier some planets were found around this one, but in a weird pattern. It was previously conjectured that they were a couple planets sharing an orbit, presumably on the Trojan / Greek plan. But the authors used the CHEOPS telescope and sussed it out: there are six of them and several share a resonance. Here's the arxiv.

Specifically the orbits' periods are: 1.91, 3.24, 6.56, 9.96, 15.23, 20.71 days. Those last five are in Laplace, 2:4:6:9:12. This is a stable one so we're assuming TOI-178's planets remain in that configuration should we use the Alcubierre drive to visit them today.

"TOI" stands for "transit" so we not only have their mass, we have their radius (at least to the cloud layer) and density. Also some of the transits interfered with one another which is how we got their resonance sorted.

These fine fellows have estimated the temperatures of the planets in question. Obviously they're all very close to their star, cool as that star is. Indeed the innermost is 1040 K and outermost 400 K. For reference Mercury runs 90-740 K, on account it has no atmosphere and its nights are long due to rotation. Earth should be 255 K but, hooray for Eemian / Holocene interglacials.

As masses go, they're all bigger than Earth. The innermost two are Earthlike in density. The others are much less dense.

I think some of the TOI-178 inner planets may end up tidally-locked similarly to Mercury. The outer planets should be Venuslikes but, like, more so. Looking especially at the third planet "d", 2.57 Earth radius and 0.177 density (and 690 K). Compare Neptune 0.29 or Uranus 0.23. Some kind of compressed hot water ice?

As to why we should care: well, we shouldn't... directly. Indirectly the authors liken the system to the Rosetta Stone. The Rosetta Stone's content remains interesting only to Ptolemaic scholars. But its three (3) languages opened scholarship up to the whole world of pre-Coptic "pagan" Egyptology. The TOI-178 authors hope that others might use this system to tease out so-far unconstrained properties of other Laplacian exosystems.

The paper lists them: GJ 876, Kepler-60, Kepler-80, Kepler-223, Trappist-1, and especially K2-138 so far the best-observed. Trappist-1 is the one you'll have heard about since its planets are coolest - possibly habitable. Gliese 876's 1:2:4 giants (not innermost d) used to get more love from being only 15 years out, hiding in front of Skat at Delta Aquarii just below ecliptic, and (c and b) being habitable-zone. Mind, Gliese 876's set don't transit for us, inclining 53-59º as they do - hence why they're not on the paper's density chart. Although c has no moon, b might have one. Mind the flares and the planetary magnet, though.

For TOI-178 [unlike GJ 876] is hope for finding other planets further out, now that the main perturbations to this star are accounted. CHEOPS is a LEO 'scope so maybe an L2 would be better.