Exoplanet Guide

Scienceline’s Guide to the Exoplanets: Far Away

2MASS J2126–8140: Where a year is longer than an Earth ice age

June 1, 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]

Far Away
2MASS J2126–8140
Discovered by: Cruz, Kirkpatrick and Burgasser (sort of)
Discovered: 2009 (sort of)
Distance from Earth: ~90 light-years
Mass: ~3700 Earth masses (~11.6 Jupiter masses)
Surface climate: Death
Habitability for humans: No

Last time, we looked at an exoplanet that orbits so close to its star that a full circle, a year on its surface, would pass in the course of a morning on Earth. This time, we’re looking at — well, the exact opposite of that. We’re looking at the largest orbit discovered to date.

This object bears the (truly evocative) name of 2MASS J2126–8140. A year ago — when 2MASS J2126–8140 was last at the current point in its orbit — Homo erectus still roamed the Earth. 

A year on 2MASS J2126–8140 could quite literally last a million Earth years. Even Pluto will complete some over 4000 orbits before 2MASS J2126–8140 completes even one.

You’d expect a planet with that long a year to have an equally colossal distance from its star, and you wouldn’t be wrong at all. Indeed, not only does 2MASS J2126–8140 have the longest orbit of any known exoplanet, it also has the biggest. 2MASS J2126–8140 gently arcs through space more than 4500 AU from its star: half a trillion miles. 

Were 2MASS J2126–8140 in our solar system, it would be ten times more distant than the hypothetical deep-space Planet Nine, and a hundred times more distant than Pluto. That’s almost a tenth of a light-year; it would take the better part of a month for light from its red dwarf parent star to reach it.

Technically, there’s some blurriness to what 2MASS J2126–8140 actually is. At about 13 times the mass of Jupiter, it touches the limits of the largest possible planets. When astronomers first found it from sky survey data in 2009, they thought it was a brown dwarf, a failed star existing in the border regions between the largest gas giants and the smallest stars. Furthermore, they thought it was a rogue object, free-floating through space.

Only in 2016 did astronomers find that 2MASS J2126–8140 is moving at almost the same velocity as and almost the same direction to a nearby red dwarf, the equally evocatively-named TYC 9486-927-1. The chances of this happening naturally are, to put it one way, astronomically low; the astronomers thought it far more likely that they’d actually found the widest star system known thus far.

So what is 2MASS J2126–8140 actually like? Is it a cold, frozen world of lonely gases doomed to orbit in perpetual darkness for billions of years? Well…not quite. Astronomers actually believe that the world is quite hot: probably over a thousand degrees in temperature, thanks to the world’s internal heating.

Brown dwarfs, after all, are stars that just weren’t able to start fusion.

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