Gliese 9827 d, and e

Gliese 9827 at about 30 parsecs had its planetary system found in 2017ish, in that K2 extension of the then-ailing Kepler. It is K6V, cooler and lower-mass than our Sun. The outer one is "d".

Their periods are 1.2, 3.6, 6.20186; so, yeah, the inner b:c are nigh a 3:1 resonance. c:d is, what, 31:18? 16:9? - maybe even 3:2. The authors did not want to call it all a resonance. Still: the star is over five Gy old, so cannot be exerting all that much saecular drift anymore. (The authors use an older estimate, to approximate six Gy; for our purpose, that's unimportant.) Laplacians seem to form early, like HD 110067. If GJ 9827 isn't a 9:3:2 Laplacian how else could it survive? - "read on", I suppose.

The system is close to us and, of course, a transit. The outermost GJ 9827 d is a 3.42 M_⊕ superVenus, or subNeptune. It might still have an atmosphere, so was marked as a candidate for spectrography. Now Pierre-Alexis Roy and Björn Benneke (et-al.) reveal: there's hydrogen on it.

Now: the constraints aren't great after only eleven passes (besides assuming tidal-lock). No reflected infrared has been measured, at all. The temperature estimate assumes black planet, which is laughable - 680 ±25 K, for all we care. The paper instead finds transit depth variability. Eleven of d's years is, what, seventy Earth days; enough to observe a ∼30 day fluctuation in the starlight. This may reflect another planet but they're not willing to say that yet. They ponder, rather, starspots. The fact of a 1.2 day "b" planet, to my view, has to be doing stuff to this star's innards. A starspot map would help.

In advance of fine-tuning d's atmosphere even temperature, two theories are possible, hydrogen or water. After 5-6 Gy no Venus-zone planetary mass should have held its primordial hydrogen. It might have tacked into this tight orbit recently but, why now? Otherwise: water vapour, above whatever clouds it has. Benneke tells the media of both scenarios, but his coauthors in the paper agree upon free hydrogen as unlikely.

Although at that mass it could keep what volatile gases it formed with: the water is hard to square with a rock-layer formation. So either way, they think, this thing formed further out - 2 AU should probably do it, for ices in this system - and tacked in. Also might explain its nonLaplacian orbit.

Tacking-in suggests that d passed its orbital momentum to something else. I suspect the system sports at least one larger planet out there, but not 30 days: instead sailing further than 2 AU from the inner three.