Distant planet's exposed core found

The skeleton of a planet, floating in space
14 July 2020

Interview with 

David Armstrong, University of Warwick


A large astronomical telescope against a dark starry sky.


Astronomers have discovered what looks like the skeleton of a giant planet - one that should be a gas giant like Jupiter, but with all its gas lost into space. Using a number of telescopes, the international team peered hundreds of light years out into space. Their discovery has the catchy name of ‘TOI-849-b’, and it’s probably made of rocks similar to the Earth, but it’s forty times larger, making it the biggest ‘terrestrial’ planet ever found. Phil Sansom heard about it from the University of Warwick’s David Armstrong…

David - We've discovered the most massive terrestrial planet that's been found to date. It's the most massive rocky planet by a long way.

Phil - When you say terrestrial, what do you mean? Is that - that's not Earth out there?

David - No, it's not like Earth, but it does have a density very similar to Earth's, which means we think it's composed of similar materials like rocks and heavy elements.

Phil - How far away is it? Where in the sky?

David - It's in the Fornax constellation in the Southern hemisphere. So us in the UK won't be able to see it in the night sky, but it's about 730 light years away from Earth. And we detected it with a NASA test mission, which is observing lots of stars, trying to look for periodic small dips in light as planets pass between us and the stars.

Phil - Now, if all you can tell about it is that it's making the star it goes in front of a little bit dimmer, how can you tell any of this stuff about it being rocky?

David - Well, in terms of how much dimmer it makes the star, that can lead us to work out how big the planet is. Because the bigger the planet is, the more of the starlight it blocks. Now that doesn't tell us the composition. We have to follow it up with other instruments to try and work out how massive the planet is. The primary one is the HARPS spectrograph, which is at the La Silla observatory in Chile. So that measures the star and takes its light and spreads it out into lots of different colours, the wavelengths we call them. And in those colours, there's a very specific signature that's unique to the star. If we take multiple images like that and look at these signatures and how they shift over time, we can see it shift back and forward.

Phil - What do you mean that the colours shift?

David - Much like an ambulance going past you, if you hear the siren pitch change, when it comes towards you and moves away from you, the starlight does the same thing as it moves towards and away from us. Now this isn't moving very fast. And so the speed is really something like a brisk walking pace, but because of modern techniques and instruments, we're still able to detect that.

Phil - That's unbelievable. So you can tell these small differences in how the star itself is moving towards and away from us. But how does that tell you about the planet?

David - Because much like planets orbit stars, the star actually orbits something called the centre of mass of the system, which is really located close to the center of the star, but just a little bit towards the planet. Both of them orbit each other, rather than one orbiting the star only. The more massive the planet is, the faster the star moves, and we can connect those things and work out, once we know the mass of the star through other techniques, how massive the planet is.

Phil - Oh, interesting. So you're checking out the impact of the planet's own gravity on the star?

Phil - Yes, that's right. So once we've got the radius or size of the planet, as well as its mass, that lets us work out its density.

Phil - And you said it was the biggest rocky planet that you'd ever seen. How much bigger than the earth?

David - Well, most massive. It's 40 times more massive than the earth. It's about three and a half times the Earth's radius.

Phil - Do you have theories about what it's doing there? How it got to be so big?

David - To get a heavy element planet this size, is very difficult. Normally when these cores are forming, once they pass their critical mass, usually around 10 times the mass of the earth, we expect them to start building up lots and lots of hydrogen and helium very quickly. So it's actually very hard to make a planet this dense without building up lots of light gases like that, and turning into something like Jupiter. One option is that this used to be a planet, much like Jupiter and lost all of its outer gases to become the planet we see today. There are a few ways that could happen. I mean, maybe it collided with another forming planet, quite late in its formation, and that can blow away the remnant atmosphere and leave behind a dense core like we see. Another way it could be if it interacted with its host star, very violently in something called tidal disruption, where the tidal forces between the planet and the star build up energy in the planet's atmosphere and that causes it to sort of blow away and just leave the core behind.

Phil - Either way, it sounds like something really violent must have happened to leave this huge either shell or weird anomaly.

David - Yeah, that's true. I mean, the other more gentle option, if you will, is that the planet just got stuck while it was forming. And rather than building up all of the hydrogen and helium that we expect, just somehow managed to avoid that.

Phil - Hypothetically, if you could get me there, do you think I'd be able to, would I be able to stand on it? Would the gravity crush me?

David - The gravity would crush you and the temperature would incinerate you pretty rapidly as well. The planet orbits its own star in only 18 hours. So the temperature is thousands of degrees.

Phil - What did you say it was called?

David - TOI849B.

Phil - Okay. I'll memorise that on my list of destinations not to go to.


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