Wednesday, July 22, 2020

Returning to nonperiodic transits

A planetary transit gives us the diameter of a large planet. The transit also nets us distance from the star and at least a constraint on its mass... if we keep watching that star. It happens that sometimes we don't keep watch. The TESS survey kept watch only for 27 days per region, so if the gap between transits took longer than that (or maybe even than fourteen days...) we didn't catch its rate of recurrence.

After the first sweep of transits, we're now checking back up on single-transit events that didn't recur in the window.

The university of Warwick found one: K-star NGTS-11, 620 light years away. The planet "b" runs a 35.5 day period, and is 0.847 Rj. This planet in turn wobbles its low-mass star: pinning its mass to 0.344 Mj. It's only as dense as Saturn!

Given that the star is only a K, if the planet is cloudy as Saturn (or Venus) it's 435 K (160° C). That "coolth" is rare for transiting planets since, as noted, we'd been catching under-27-day transits; most of these cooler planets are locked around red dwarfs. I say "locked" because 27 days on down is... like our Moon, so should present the same face toward the barycentre.

0.344 Mj is still massive. Saturn itself is only 0.3. And although it's hot, it's not so hot nor so subject to flares as some others. And it's not dense enough to be a planet of supercritical ices - those are alternate Neptunelikes. Warwick is right, I think, to treat this as a nonexotic Saturnlike. Likely it migrated inbound to its present orbit.

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