Exoplanet Guide

Scienceline’s Guide to the Exoplanets: Fly on Light Bulb

HD2685 b: Sitting around a bright star

August 10, 2020
A fake planet with "Scienceline's Guide to Exoplanets" labeled across its equator. Other planets zoom past behind it.
Your favorite planets, and you didn't even know they existed. [Credit: Curtis Segarra | CC BY-NC-ND 2.0]

Fly on Light Bulb
HD2685 b
Discovered by: TESS
Discovered: 2018
Distance from Earth: ~650 light-years
Mass: ~375 Earth masses (~1.2 Jupiter masses)
Surface climate: Balmy
Habitability for humans: No.

Today, we’re looking at HD2685 b, another hot Jupiter: another gas giant that’s close to its host star, close enough that the gases at its surface would be hot enough to boil calcium.

In this case, it’s actually HD2685, the host star, that especially interests astronomers. It’s an F-type star: a yellowish-white star that’s only around 57% larger in radius than the Sun, and only around 44% more massive.

But HD2685 is one of the brightest main-sequence stars yet known to host a planet; it’s over four times as bright as the Sun. Of course, there are brighter stars with planets, but those tend to be giant stars in themselves.

We don’t often find planets around very bright main-sequence stars, for a number of reasons. For one, there are simply more low-mass, dimmer stars in existence; the very bright blue-white stars at the upper edge of the main sequence are in reality fairly rare. For another, those blue-white stars are also very hot, and scientists think they can blow away their stellar discs before they can form proper planets.

At any rate, HD2685 b orbits at a very modest 0.05 AU: round about eight million kilometers, or five million miles, from its star. At that distance, the equilibrium temperature of HD2685 b’s surface would hover around 2000 Kelvin, or around 3140 degrees Fahrenheit. Not only is that searing hot, the star would utterly dominate the view from the surface.

You might expect HD2685 b to warrant no more than just a simple observation before being cast into the astronomy footnotes. It’s not of interest for those seeking other Earths, and it’s both relatively distant and modestly sized for hot Jupiters in the first place. After all, an incredible number of hot Jupiters are known to exist — as, due to their relatively large mass and their close distance allowing them to blot out light, they’re some of the easiest planets for Earthbound astronomers to find.

But you’d be wrong, and all because HD2685 is so bright.

The astronomers who discovered HD2685 b state that it’s an interesting candidate for follow-up observations, in large part thanks to its impressive brightness. And it’s also a fairly young star, at only around a billion years old. It’s entirely possible that HD2685 b is still in the early stages of its life.

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