Space Matter: The Planet KELT-9b Is Hotter Than a Star

Science Features Discovery
Space Matter: The Planet KELT-9b Is Hotter Than a Star

Yes, you read that right. About 620 light years away, there’s a planet called KELT-9b that’s hotter than many of the stars in our galaxy.

How is that possible?

According to a study published in the journal Nature, the amount a planet is irradiated is dependent on the temperature of the star it orbits. That makes sense—the hotter the star, the hotter the planet (all else, such as distance from the star, being equal.)

Now, A-type and B-type stars are much hotter stars than our Sun. While A-type stars have a surface temperature range between 7,600 and 11,500 K, B-type stars run from 10,000 to 30,000 K. For comparison, the surface of our Sun is a comparatively cool 6,000 K. A- and B-type stars are also much more massive than our own star.

The Nature study specifically focuses on the planets that orbit these incredibly hot stars. As of June 22, 2017, we’ve discovered 3,497 confirmed exoplanets, or planets outside our solar system. A mere eight of these planets orbit A-type stars. We have found no exoplanets orbiting even hotter B-type stars.

1.summertriangle.pngThe Summer Triangle is composed of three A-type stars: Vega, Altair, and Deneb (Image credit: Raffaella Mattei Cattani)


This doesn’t necessarily mean that there are fewer planets around hot stars, though; the fact is that it’s easier to find exoplanets around comparatively cooler stars like our Sun. One way we confirm that a suspected exoplanet is an actual one is through Doppler spectroscopy; until 2014, it was actually the most widely used method. Scientists can see shifts in a star’s radial velocity with respect to earth (the speed at which the star moves away from us as the universe expands) that occur as a result of a planet’s gravity acting on the star using a spectrometer. Hotter stars don’t have as many spectral lines as cooler ones, which makes confirming exoplanets around these stars much more difficult.

KELT-9 is a A-type star with a temperature of around 10,000 K and a mass over double that of our Sun. It’s the hottest star known to have a planet orbiting it, called KELT-9b. Now, KELT-9b isn’t an Earth-like planet. It’s a massive gas giant, similar to Jupiter, and it takes just 36 hours to complete one orbit of KELT-9. That’s because of its unique orbit—as seen from the Earth, it orbits at its star’s pole, rather than around the equator, as occurs in our solar system (well, except for Neptune and its weird elliptical orbit).

2.kelt9b.pngAn artist’s concept of KELT-9b around its host star. (Image credit: NASA/JPL-Caltech)


The fact that KELT-9b is so close to its host star also means that the planet is tidally locked, like our Moon is to the Earth. We only ever see one side of the Moon; its orbital period of the Earth is equal to the amount of time it takes the Moon to rotate on its axis (24 hours). Similarly, a day on KELT-9b and a year on KELT-9b are both the same amount of time — 36 hours.

The fact that one side of the planet always faces KELT-9 is significant for the planet’s temperature. There’s no possible cooling off period for the planet. And as a result, it’s hot — hotter than any other planet we’ve discovered thus far. Its temperature? 4,600 K, or around 7,820 degrees F.

Why hasn’t this planet turned into a star, then, if it’s actually hotter than stars? It’s a good question, and the answer goes straight to the heart of why and how stars form. The catalyzing issue when it comes to star formation isn’t temperature; it’s mass. In fact, it’s likely that our own Jupiter is actually a failed star; a gas giant must have around the mass of 13 Jupiters to be considered a star (and it would be the coolest of stars, a brown dwarf). That’s the mass at which the fusion of deuterium is possible (unlike regular stars, brown dwarf stars don’t have the mass to fuse hydrogen in their cores).

3.jupiter.pngThe storms of Jupiter’s south pole, as photographed by Juno (Image credit: NASA/JPL-Caltech/SwRI/MSSS/John Landino)

The bottom line, then, is that it’s possible for KELT-9b to be hotter than a star because a) it orbits an incredibly hot star b) very closely. It’s so close to its host star, in fact, that its c) tidally locked. The side that faces the star is, thus, superheated.

Isn’t space cool?

Top photo courtesy of Robert Snache

Swapna Krishna is a freelance writer, editor and giant space/sci-fi geek.

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