<p>It's because it is a gas giant (arguably it should be called a brown dwarf) and its temperature is not governed by the radiation it passively receives from the star it orbits. The heat is generated by gravitational contraction - an ongoing process since the planet was &quot;born&quot;. Half the gravitational potential energy released by contraction goes into heating the planet/brown dwarf and half is lost as radiation into space.</p>
<p>Indeed, as the Wikipedia article mentions, there are isolated Y-type brown dwarfs that have a similar temperature, and for the same reason. See for example <a href="https://astronomy.stackexchange.com/a/8128/2531">https://astronomy.stackexchange.com/a/8128/2531</a> and the plot below which shows the cooling history as a function of mass (adapted from Burrows 1997 by Philip Muirhead).</p>
<p><a href="https://i.sstatic.net/UDdCO19E.png" rel="noreferrer"><img src="https://i.sstatic.net/UDdCO19E.png" alt="Evolutionary tracks Teff vs time" /></a></p>

CC BY-SA 4.0
2024-07-26T11:28:57.260Z
Why doesn't Jupiter have this own temperature while it's also a gas giant?
Snack Exchange
CC BY-SA 4.0
2024-07-26T12:13:48.843Z
@SnackExchange it does. Jupiter radiates more energy than it receives from the Sun. But a lower mass gas giant cools more quickly.
ProfRob
CC BY-SA 4.0
2024-07-26T09:55:53.640Z
2024-07-26T12:30:33.337Z
2024-07-26T12:30:33.337Z
ProfRob
<p>I was reading the Wikipedia page for the exoplanet <a href="https://en.m.wikipedia.org/wiki/Epsilon_Indi_Ab" rel="noreferrer">Epsilon Indi Ab</a>.</p>
<p>It orbits around its parent star every 173.6 years in a distance of 28.4 AU as provided in the first infobox of the article.</p>
<p>The parent star is in fact a star system consisting of three stars called <a href="https://en.m.wikipedia.org/wiki/Epsilon_Indi" rel="noreferrer">Epsilon Indi</a>. The stars are Epsilon Indi A, Ba, and Bb respectively.</p>
<p><em>Epsilon Indi A</em>, the star that the mentioned exoplanet revolves around has a surface temperature of only 4649 K which makes it even colder than our Sun.</p>
<p><strong>My question</strong>: How does an exoplanet orbiting around a cold star at such a huge distance have a temperature of near 275 K?</p>

<star><orbit><exoplanet><temperature>
Why is Epsilon Indi Ab 2° Celsius?
Snack Exchange
# Why is Epsilon Indi Ab 2° Celsius?

(Asked by: username_0 on 2024-07-26)

I was reading the Wikipedia page for the exoplanet [Epsilon Indi Ab](https://en.m.wikipedia.org/wiki/Epsilon_Indi_Ab).

It orbits around its parent star every 173.6 years in a distance of 28.4 AU as provided in the first infobox of the article.

The parent star is in fact a star system consisting of three stars called [Epsilon Indi](https://en.m.wikipedia.org/wiki/Epsilon_Indi). The stars are Epsilon Indi A, Ba, and Bb respectively.

*Epsilon Indi A*, the star that the mentioned exoplanet revolves around has a surface temperature of only 4649 K which makes it even colder than our Sun.

**My question**: How does an exoplanet orbiting around a cold star at such a huge distance have a temperature of near 275 K?

**Tags:** star, orbit, exoplanet, temperature

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## 1 answers

### Answer by username_1 on 2024-07-26. Score: 9 | Accepted answer

It's because it is a gas giant (arguably it should be called a brown dwarf) and its temperature is not governed by the radiation it passively receives from the star it orbits. The heat is generated by gravitational contraction - an ongoing process since the planet was "born". Half the gravitational potential energy released by contraction goes into heating the planet/brown dwarf and half is lost as radiation into space.

Indeed, as the Wikipedia article mentions, there are isolated Y-type brown dwarfs that have a similar temperature, and for the same reason. See for example <https://astronomy.stackexchange.com/a/8128/2531> and the plot below which shows the cooling history as a function of mass (adapted from Burrows 1997 by Philip Muirhead).

[![Evolutionary tracks Teff vs time](https://i.sstatic.net/UDdCO19E.png)](https://i.sstatic.net/UDdCO19E.png)

> **Comments:**
> **username_0**: Why doesn't Jupiter have this own temperature while it's also a gas giant?  
> **username_1**: @SnackExchange it does. Jupiter radiates more energy than it receives from the Sun. But a lower mass gas giant cools more quickly.  

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