First Author

Stars tend to be the hot bodies in the Universe. But now, scientists have found a very hot planet. On page 691, Joseph Harrington, a planetary scientist at the University of Central Florida in Orlando, details an extrasolar planet that reaches a sweltering 2,300 Kelvin (K). To get to that temperature, the planet — called HD 149026b — must absorb all the starlight that reaches it. Nature asked Harrington what this and other space oddities can teach us.

Why is this hot planet so intriguing?

Scientists predict a planet's temperature from how much starlight it absorbs. The predicted temperature for this planet is about 1,400 K and black planets tend to be around 1,700 K. But we measured 2,300 K on the day side, which is a shocker because every other exoplanet has followed the usual predictions. Observing weird things is like striking gold because it means there is something more to learn. Maybe the day side is hotter than the night side, but if so, why isn't that true for all exoplanets? HD 149026b contains more heavy elements than our entire Solar System, so its chemistry might allow it to absorb starlight better than other exoplanets.

Why do you think the planet is so hot?

Our simple model suggests that HD 149026b is a black planet. It absorbs all the light that falls on it, but quickly re-radiates that energy as infrared light. The planet is therefore hot enough to emit its own light — projecting a visible orange-red glow from the area closest to the star. The picture is quite tolkienesque. This, and other evidence, makes us think that hotter planets' temperatures might differ between the day side and night side. A planet we observed last year, υ Andromedae b, is 1,400 K hotter on its day side than its night side. Another planet, HD 189733b, was shown last year to have a more modest day–night temperature difference. A picture is starting to emerge, but proof requires more work.

What will studying exoplanets tell us about Earth and the Universe?

Every time we look at a familiar thing in an exotic environment, we learn a lot. We live on a planet with some really nice conditions, and our actions affect the planet. We can learn how our planet works, such as its changing climate, by learning what happens on other planets. Also, everyone wants us to find life beyond Earth. HD 149026b is too hot for liquid water — the main requirement for life — but today's young people can expect that the conditions for life will be found on an exoplanet during their lifetimes.