Photoeccentric effect

Last weekend I caught David Kipping's youtube on the "photoeccentric effect", floated over a decade ago but impractical at the time. Kipping's crew has lately applied this upon a necessarily-small sample of M and K transiting systems, which stars could have their densities directly measured.

The notion is to compare the real stellar densities to densities inferred from planetary orbits. The latter inferral comes with assumption of orbit eccentricity zero. Where the latter density is, er; wrong: how wrong, tells us the eccentricity.

This was all moot until we could get real stellar densities for the far-off stars in question. Before the Gaia data-releases (DRx) we often didn't have good parallax for how far-off; if you recall Whatmough's "Extrasolar Visions", it was stuck with the wrong distance for at least 70 Virginis. Since then, we have Andrew Mann et al. 2019. This concocted a method from 63 nearby binaries; unfortunately only up to 70% Sol mass (I take it that above that, like with our G-type Sol, we are tripped up by stellar age and/or second-generation effects). Also a signal:noise contraint applies although I don't see where Kipping's team used that exact paper.

Rather than constrain by mass they went with the eclipse radius, up to twice Earth's. Such're probably superEarths. But even Musk isn't hoping to travel to a literal Kepler Object-of-Interest (KOI) or TESS-O.I. These are proxies: for, yes, Earths; albeit more for Veneres since the research allowed insolation up to four Earths.

I'd actually thought that eccentricity could be measured directly. I take it that I've been living in the late 1990s when the star's radial velocity was harshly perturbed over long light-curves. That was seen with such planets, say, as 70 Virginis b. But when we're talking sub-Neptune size, their mass doesn't budge the star as much and - for close orbits like transits - the light-curve is short. We might not even have a good idea of the planet's mass, let alone eccentricity. The photoeccentric effect - lovely name by the way - does the constraining, at least for e.

It was found that of the seventeen test-cases, all but KOI 4087.01 orbit with low-eccentricity: e ≤ 0.1. For reference Mars has 0.09; Earth is - you know, Earth, and Venus has hardly an ellipse at all. Habitability doesn't really become a problem until 0.3 - they claim that the summers however short will boil off the seas. Which, yeah, can't have helped Mars' seas.

If these can serve as proxies, Earthlike planets in the HZ - at least a K's HZ - mostly didn't migrate far. As not being boiled, I've expected that each K HZ bears mainly high water planets, like Trappist-1 efg on the M side.