Exoplanet Guide

Scienceline’s Guide to the Exoplanets: The Lonely Colossus

HD 106906b: The gas giant that shouldn't exist

March 2, 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]

HD 106906b
Discovered by: Vanessa Bailey, et al.

Discovered: 2013
Distance from Earth: ~337 light years
Mass: ~11 Jupiter masses (~3500 Earth masses)
Surface climate: Hot gases sure are fun, aren’t they?
Habitability for humans: No.

In the multiverse of Doctor Who, the titular Doctor hails from the planet Gallifrey, home of the Time Lords. Before it was disrupted by a space-time-spanning war, Gallifrey was an ancient world of political intrigue and more political intrigue and even more political intrigue. Gallifrey is also a rather massive planet a bit over 300 light years in the direction of the constellation Crux.

Well, rather, it’s probably not there, but over 100,000 people did sign a petition to give the name “Gallifrey” to the exoplanet HD 106906b.

HD 106906b is…odd. It’s very big — at 11 times the mass of Jupiter, it’s not far off the range where a gas giant stops being a gas giant and starts being a sort of shadow star called a brown dwarf. It also orbits a binary pair of two nearly identical stars, both just a little massive than Earth’s Sun.

But what really caught planet-watchers’ attention was just how far removed from its suns the planet was: over 700 AU. That’s over 100 billion kilometers, or over 60 billion miles — nearly 25 times the distance from Earth’s Sun to Neptune.

It’s so staggeringly far that astronomers were initially unsure if HD 106906b orbits its suns at all, or if they’d just happened to capture a rogue planet in the right place at the right time. This is possible, but astronomers now consider it very, very unlikely.

You might expect a planet that far out to be as frozen-cold as the deep space surrounding it. HD 106906b is quite the opposite, actually. At the surface, it’s a pleasantly tropical ~1600 degrees Celsius, or ~2900 degrees Fahrenheit. This is because HD 106906b is very, very young: only a few million years old. The planet is still carrying over the residual heat of its birth.

In fact, HD 106906b’s system still has its protoplanetary disc: the dusty shroud of debris from which planets and other objects form. Adding to the general weirdness of this system, astronomers have found the disc to be wildly asymmetrical — it extends four times as far out on one side than on the other.

Scientists aren’t yet sure what HD 106906b’s orbit actually looks like. Neither are they sure how any of this formed. Planets as massive as HD 106906b shouldn’t exist that far away, not when their system is still forming and there hasn’t been much time for them to move out.

It’s been theorized that HD 106906b formed independently of its suns — as if it were a star that just never was. More recently, modeling has suggested that the planet may have formed much closer, only to be shoved out by an interaction with its binary suns and stabilized by a passerby, such a nearby third star.

Or, perhaps, it was Time Lord astroengineering.

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