The Hottest Known Exoplanet Is (Probably) Slowly Melting Away

6:05 minutes

An artist’s concept of planet KELT-9b, a planet hotter than most stars. Credit: NASA/JPL-Caltech

The community of exoplanet hunters is more often focused on finding potentially habitable worlds around dimmer, cooler stars. But what happens when you turn your telescope to a bright, hot star whose planets might never support life?

Meet KELT-9b, an exoplanet that researchers reporting in Nature say is twice the size of Jupiter. With a temperature of over 4,600 degrees Kelvin, it’s hotter than most stars—and the hottest exoplanet found to date. Moreover, it’s probably very close to its star and tidally locked, with one side constantly in daylight and losing matter to evaporation, says study co-author and Ohio State University astronomy professor Scott Gaudi.

What can we learn from conditions this harsh? Gaudi explains.

[Seven new chances for life in space, just 40 light-years away.]

Segment Guests

B. Scott Gaudi

B. Scott Gaudi is a professor of Astronomy at the Ohio State University in Columbus, Ohio.

Segment Transcript

IRA FLATOW: Yet again we have found another exoplanet. And if you hoping to live there, not such a good idea. It’s 4,600 Kelvin– that’s the heat. It’s the hottest– well, it’s hotter than most stars. And it’s the hottest exoplanet we have found to date. And no life as we know it could survive in that furnace. Even the molecules there would melt into atoms. So it’s sort of a gassy planet. That’s just a fact of life on KELT-9b, an exoplanet twice the size of Jupiter. And as I say, the hottest planet they’ve found to date.

In writing for Nature this week, astronomers describe a planet stuck close to its bright massive star and blasted by intense radiation to the point where it may actually be slowly melting away. Here to take us on a quick tour of this hellish landscape is my guest Scott Gaudi, professor of astronomy at Ohio State University in Columbus, Ohio and co-author of the research. Welcome to Science Friday.

SCOTT GAUDI: Thank you for having me.

IRA FLATOW: This has got to be a setup to a joke, you know. How hot is it, you know?


SCOTT GAUDI: It’s pretty unbelievable. Yeah, I mean I could imagine this being the punchline to a joke. It was certainly kind of a punch in the gut when we discovered this planet and finally confirmed that it was real. In fact, for many years– I mean, we found the first signal of this planet back in 2014. And it has taken us almost three full years to confirm it and finally publish it. Part of that was just because we had a hard time believing what we had found.

IRA FLATOW: So how does a planet get so hot and such high intensity and so close to it’s sun?

SCOTT GAUDI: Well so, I’ll answer the last part in a little bit– how did it get so close to its star? That’s a question we don’t really have a good answer to. We have some theories. Why is it so hot? Well, two reasons, mainly.

One is because the host star itself is extremely hot. The host star itself is what astronomers call an A star. A stars that you may have heard of are Vega and Sirius and Altair. They are stars that have temperatures of roughly 10,000 degrees Kelvin. And because they’re so hot, they’re extremely luminous– that means they put out a lot of photons per second. And they also tend to put out very high energy radiation. So they would appear– and they do appear– if you go out and look in the night sky at Vega or Sirius, they appear blue to our eyes. So this host star is putting out a lot of radiation.

And then that’s compounded by the fact that the planet is on a very short orbit. It’s only about one and a half days. So its year is about 36 of earth hours. And it’s only about three stellar radii from its parent star.

And then finally, the planet is so close to its parent star that it is tidally locked. That means that, similar to our moon is to the earth, that means that one side of the planet is always facing the star. And the other side of the planet is facing the cold night. And so that one side of the planet is constantly being bombarded by this intense, high energy radiation from its host star. And that raises the day side temperature of this planet up to only a few thousand degrees cooler than the sun and hotter, as you say, than most stars.

IRA FLATOW: This is Science Friday from PRI, Public Radio International, talking with Scott Gaudi, astronomy professor of Ohio State University who said he felt a punch in the gut when he discovered KELT-9b. Does that mean the other side is really cold? While this one side is melting, the other side’s freezing?

SCOTT GAUDI: This is an excellent question. We don’t know entirely. But our guess is that the other side– the night side of the planet is much cooler than the day side of the planet. It’s probably not zero because the planet– obviously we know from our elementary physics that if you put something hot next to something cold, the heat likes to travel from hot to cold. So the planet is trying its best to create a uniform temperature, but unfortunately its radiating at such a high rate that it’s unable to do so.

So the day side really is much hotter than the night side. The night side is still pretty hot. It’s the temperature of, say, red dwarfs. So still the temperature of a star, but several thousand degrees, likely, cooler than the day side. That’s a hypothesis though, but it’s one that we can test using Spitzer space telescope. And we hope to.

IRA FLATOW: Can you learn anything useful from studying such a rogue planet like this?

SCOTT GAUDI: Sure, I mean I think whenever you find these extremes that nature provides for us, it helps test our theories. So I’ll give you a quick example of this.

When I first found this discovery, I talked to a friend of mine– Jonathan Fortney at University of California, Santa Cruz– and asked him to model it. He’s an expert at modeling these kinds of highly irradiated giant planets. And he wrote back to me a few hours later and said that this system actually broke his code. His code crashed because it was so extreme. So when we find these extreme systems, they test our understanding of the physics. And sometimes the physics works, and we gain more confidence. Sometimes our codes break. And so we have to go back to the drawing board and really study the physics of these systems.

IRA FLATOW: I bet you love that part of it. This is so fascinating, Dr. Gaudi. Thank you for taking time to be with us today. We’re going to follow this a little more, OK?


IRA FLATOW: We’ll have to have you back to talk more about it. Scott Gaudi, astronomy professor at Ohio State University and co-author of this paper on KELT-9b.

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About Christie Taylor

Christie Taylor was a producer for Science Friday. Her days involved diligent research, too many phone calls for an introvert, and asking scientists if they have any audio of that narwhal heartbeat.

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