Astronomers have found a weird planet orbiting a very Sun-like star. Called K2-360b, the planet is 1.6 times Earth’s diameter, but 7.7 times its mass. That makes it dense: with an average density of 11 grams per cubic centimeter, it’s twice as dense as Earth! That’s literally as dense as lead. That mass and size makes its surface gravity three times Earth’s. If you could stand on it (which I would not recommend for many reasons, as you’ll see) you’d weigh three times what you do here.

The host star is K2-360, a rather amazingly Sun-like star with almost exactly the same mass and size as the Sun. It’s likely about 6 billion years old, so a bit older than our spry 4.6 billion-year-old star. 

The planet was found in data from the K2 mission, which was originally Kepler, a space telescope that scanned the sky measuring the brightnesses of stars [link to journal paper]. If a planet orbits the star and we happen to see that orbit edge-on, then once per orbit the planet passes in front of the star, creating a mini-eclipse called a transit. The amount of light blocked and duration of the transit tells us the size of the planet and how long it takes to orbit.

K2-360b is in tight with its star, orbiting it once every 21 hours. That means it’s only about 2 million kilometers above the surface of the star, so it gets baked. The temperature of the planet is probably around 1,800°C, hot enough to melt iron! So yeah, you don’t want to visit.

Its size makes it a super-Earth, an exoplanet less than about twice Earth’s diameter. Why is it so dense? It likely started off life more like Neptune, with a thick atmosphere, but over time its ridiculously close orbit to the star meant its atmosphere got stripped away by the intense heat. What was left was the rocky-metallic core. The astronomers who did this research ran some models of what the interior is like, it’s likely to be about 50% core, which would explain the density; the cores of such planets are mostly metals. Fun place.

The mass was found by seeing how hard the planet tugs on its star as it orbits. This is called the radial velocity method: the star and planet orbit a common center of mass, so the star is making small circle as the planet makes a wider one. Sometimes the star is headed toward us and sometimes away, so there’s a Doppler shift in its light that can be measured and used to determine the planet’s mass. 7.7 Earth masses is a lot, and having it crammed into a planet only 60% wider than Earth is astonishing.

The radial velocity measurements indicate there’s likely a second planet in the system as well, though it doesn’t transit. The astronomers found it’s about 13 million km out from the star in a 9.6-day orbit. It has 15 Earth masses, making it more like Neptune and Uranus. It’s still hot, but not nearly under such a blowtorch as the inner planet.

Planets like these are important. They show us what incredible diversity nature has in making worlds, but it also tells us how planets form and change over time. We know planets that get close to their stars must form farther out and migrate inwards; when this system was still forming, the protoplanetary material that close to the star could never aggregate together to form a big planet. We know the planets in our own solar system underwent a lot of migrations early on, but likely nothing as severe as what we see with these incredibly hot exoplanets. 

And one of my favorite outcomes of finding planets like this is perspective. It’s easy to call these planets weird or oddballs, but we keep finding them. It makes you really wonder if we’re the weird ones.

As much as I’d like to avoid asteroids hitting Earth, I kinda love it when they hit Mars. Nobody lives there, so I only feel a little guilty that our ochre-colored neighbor gets slammed more than we do… after all, it’s closer to the asteroid belt, and its much thinner atmosphere means more space rocks make it to the ground.

Even better: we have orbiting eyes in the sky that find these impacts and send back amazing images of them!

This image was taken by the European Space Agency’s ExoMars Trace Gas Orbiter. The impact happened in February 2021, leaving behind a 21-meter crater. The feathery lines around it are called rays, and are plumes of material excavated out in the impact and blasted many hundreds of meters away. These are common on the Moon, too, and indicate a young impact; the rays wear away rapidly on a geologic timescale.

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