Exoplanet Guide

Scienceline’s Guide to the Exoplanets: The Distant Point of Light

GU Piscium b: A very large world that twinkles

October 26, 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]

The Distant Point of Light
GU Piscium b
Discovered by: Marie-Eve Naud et al.
Discovered: 2016
Distance from Earth: ~155 light-years
Mass: ~11 Jupiter masses
Radius: ~1.3 Jupiter radii
Surface climate: Hot gases!
Habitability for humans: Bad.

Around a variable star in the direction of Pisces, and orbiting that star somewhere around 2000 times the distance from the Earth to the Sun, is this planet.

Even though its orbit isn’t the largest we know — that label belongs to another world that’s been discussed in this space before, 2MASS J2126-8140 — GU Piscium b is still respectably distant from its star. An orbit on the surface of this massive planet lasts 163,000 Earth years. A year ago on that planet’s surface, Earth was mired in the ice age before last.

GU Piscium b is a gas giant, and a truly colossal one at that. At around 11 times the mass of Jupiter. GU Piscium b starts pushing up against the boundary of where gas giants end and where brown dwarfs begin. (If that mass is correct, then the planet is likely a very large gas giant; brown dwarfs aren’t thought to begin until around 13 Jupiter masses. But, given a few discrepancies in measurement, the possibility is very much there.)

With the planet being fifty times further than Pluto, and far, far beyond the line where water would freeze, you might expect a world that’s frigid and frozen over — the gas giants of our solar system, after all, aren’t precisely warm.

But scientists think the gases at its surface are around 1000 Kelvin, or over 1300 degrees Fahrenheit; even hotter than Venus.

The planet is not a hot Jupiter; it’s far too distant for that. Instead, what drives its heat are processes of internal heating. It’s still too cool to be any form of star, but were the planet several times larger, those processes would start to fuse atomic nuclei and turn it into a star.

But perhaps the most interesting fact about planets like GU Piscium B — massive and distant from their host stars — is how they were discovered. Most exoplanets are discovered indirectly, by the effects they have on how we see their host stars here on Earth.

But because GU Piscium b is so massive, and because it’s so distant from its star, astronomers don’t need to do that. They can see it as its individual, distinct point of (reflected) light in the sky.

And there is what makes GU Piscium b more interesting than 2MASS J2126-8140. The angles and orbits align such that, when viewed from Earth’s perspective, GU Piscium b is the planet that is the largest angle in the sky away from its star.

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