Anti-Helliconia

From Yale: "The search for more temperate Tatooines". Actually a seasonal Tatooine; the anti-Helliconia. A planet orbits a primary star, and a secondary (smaller) star orbits the lot. This is from Malena Rice, Konstantin Gerbig, and Andrew Vanderburg: The Orbital Geometries and Stellar Obliquities of Exoplanet-hosting Multistar Systems. Which they didn’t paywall! yayaaaayy

The paper is all about dynamics, so forming the systems isn’t a problem – this is the paper which defines such systems! Habitability on the other hand . . .

My first proviso is that the paper concentrates on transits, a sample of 184 systems subsequently restricted to forty (reserving the right to boost the sample-set again). So: they’re close-in to the star. Leave aside habitable-zones for now…

The eight systems of interest to the paper have spin-orbit and orbit-orbit alignment. The binaries are: Qatar-6, CoRoT-2, DS Tuc, HAT-P-1, HAT-P-22, HD 189733, and TrES-4; and V1298 Tau, for ternary (spin-orbit). This led the authorial ternary to add binary TrES-2, as although too faint for the 40-system sample, it looks to share qualities with the other seven. Binary separations are like 200+ AU; that third member of V1298 Tau was a “proxima” at 10,515 AU. All this means the secondary star is not providing much light let alone heat. Might as well be Proxima v. AB – hardly proximal, from the perspective of that one’s planet(s).

So Yale’s take is clickbait as it stands.

But that is transit; just what we see. The paper's good news is that alignments cluster at stars under 6100 K. Our own Sun is 5780 K at its 4.567 Gy. So these stars are “cool”: we can talk reasonably-wide habitable-zones and long ages, types G and K. We won't see nontransits at 1 AU but we can't rule them out (anymore).

Consider a (hypothetic) M star orbiting in the Uranus range 20 AU from a G or high-K. For the first billion years the M is baking the planets inward of that; but then it cools off. How far does the M have to sail before it has no effect on warming those inner planets? How to deliver water and other “ices” to the HZ? Maybe through the eccentricity it will force upon its Kirkwoods. I worry lest the leftover ices be soots, such that the inner planets end up too carbonic.

And some major stars huddle closer. The obvious instance is the Alpha Centauri mainline, a high G with a K, the K orbiting outside the G’s inner planets. The K would have to run a bit further out than in the system αC got. Although … I hope it wouldn’t interfere with the inner planets even being created. And I expect little moisture on those planets as isn’t imported.

I further wonder if we could do this if the system had started out Helliconian: the K star and its cold planet are, together, orbiting an A. But then the A goes redgiant and dwarf. Now the barycentre has shifted (abruptly) to the K. The K has billions more years to live. Mind: in this case the dwarf isn’t delivering much radiation to the system anymore. That sounds dynamically difficult.