Hussey 918

The most wonderful person on Youtube reports on the currently-offline TOI/TESS project: it has pinned down a G system at 95 parsecs.

This tiny spot on the late-nineteenth-century photo-plates avoided the consideration of Flamsteed and Draper. William Joseph Hussey at the Lick Observatory notched "1905.17" as a "double star", publishing it March 1905 in his ninth catalogue: HU 918. It was subsequently ignored, basically; until it became TESS's "Transit Of Interest 4633". Those... ugh... "citizen scientists" precovered the data to dig into the binary's deep(er) history.

With the additional attention the binary's orbit is resolved: period 231 +32/-24 years. This is not as constrained as it could be, but in all fairness our observations pre-TESS aren't great for some tiny 95pc dotpair in a disused catalogue.

These astronomers for their part avoid the name "Hussey 918". This blog's juvenilia aside: they're unsure which star in it is the transit-of-interest. They do know which is A: it's a ~1.1 sol mass; B is 0.05 masses less. Unfortunately they are eccentric (0.91!) so they are interfering with each other's light, currently. So I will call whichever star owns the planets, "TOI 4633".

There's a Saturn(?)like b but it does not transit, so of poorly constrained mass and even volume; it remains a "candidate" pending more observation. Anton Petrov is excited about TOI 4633 c - the actual transit. This planet is 47.8 Earth mass +23.8/-27.6, which again will get better constrained dependent on stellar and "b" finetuning. Radius is better-constrained: ~2.4 Earth; contrast Neptune 3.85. Whether you call it subNeptune or superNeptune depends on whether you're talking volume or mass. Eccentricity is 0.12 a bit more than Mars'.

At this mass in its location, which rounds to 1.6 Earth flux even without that other Hussey companion: expect a large gas envelope. This will settle into a wholly obscurant clouddeck like Venus': Sudarsky II. These clouds likewise will be low on pH; although a magnetic field will keep in the water, at least. Petrov already knows no-one's living on this planet. I add: the planet's own tides will disrupt any "Doppler" world at 0.85 AU's L4/L5.

Petrov waxes more hyper about the possibility of Galilean moons. The Hill Radius would fit one; "b" is far inbound of the place. Um. Well. There's no impactor on this thing as can offer a moon like ours, of a decent proportion of this planet's mass. Titan's mass is 0.0225 Earth and that's including its shell of ice, ice which these moons won't have.

Whether these warm moons have water... there we really must speculate. How far out is the planet's magnetic shield? Are any moons Ganymedean, themselves? One can imagine a small interior moon by itself without Io-like tectonics. Or a larger moon anywhere also not being squashed, with its own dynamo. Otherwise (and most-likely) they're tiny irradiated rocks: think, Titania and Oberon.

For those who consider HU 918 important, and however good it be to constrain parameters: it might not be a candidate for Webb time - yet. Petrov considers prime time for a few more decades hence, when A and B separate sufficiently we can at least see which star is the T.-o'-I.

What this system buys us, rather: is a model of a binary system which can support a Neptunelike at Earthlike irradiance, allowing for those short hot summers of perihelion. It even offers a "hot Jupiter" inbound. Skiffy worldbuilders rejoice.

TESS 5/8: Back in business, as of yesterday.