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Astronomers Discover Exoplanet Denser Than Steel

This super-dense exoplanet packs a punch.

By Cassidy Ward

The crew of SYFY's The Ark (streaming now on Peacock) will be the first humans to ever set eyes on another star system. At least, they will be if they survive the journey. Historically, science fiction has been dominated by distant worlds and alien peoples who look an awful lot like the world and people we already know. The more we learn, however, the more we realize the universe is filled with every kind of world imaginable, and some of them are super weird.

A World Almost Twice as Dense as Earth

How to Watch

Watch new episodes of The Ark Wednesdays at 10/9c on SYFY. Catch up on Season 1 on Peacock.

Enter, the exoplanet TOI-1853b, a Neptune-sized world not too far from here. It’s circling a K2 dwarf star — a main sequence star between a conventional red dwarf and familiar yellow stars like our own Sun — roughly 85% the mass of the Sun, located roughly 545 light-years from here. Observations published in the journal Nature suggest it is denser than solid steel.

RELATED: The JWST Bagged Its First Planet, A Sweltering Earth-Sized World

The planet circles in a tight 1.24 day orbit and it’s one of the heaviest worlds we’ve ever seen. Astronomers classify it as Neptune-like because of its approximate size. TOI-1853b has a radius roughly 3.5 times that of Earth and it’s carrying roughly 73 Earth masses in its planetary pockets. For comparison, our own Neptune is about four times the diameter of Earth, roughly the same as TOI-1853b, but is only about 17 Earth masses.

An illustration of an exoplanet orbiting close to a star.

Crunching those two numbers (diameter and mass), scientists can work out the average density of a world, which can tell us about what a planet is likely made of. Earth, for instance, has an average density of 5.5 grams per cubic centimeter. The gaseous planets are usually less dense. You’ve probably heard that Saturn could float in water (provided you have a big enough glass and could solve a number of other physics problems) and that’s true. In fact, Neptune is the densest solar system planet on the other side of the asteroid belt, with a density of 1.6 grams per cubic centimeter.

TOI-1853b puts our giant planets to shame with an average density of 9.7 grams per cubic centimeter. That’s dense enough to overshadow steel, with an average density of just 8.05 grams per cubic centimeter. That’s one tough planet.

RELATED: JWST’s Neptune: The best infrared view in 30 years

To explain its bizarre conditions, astronomers suspect the planet was originally water-rich and whizzing along a highly eccentric orbit. As it whipped around its parent star, alternately closer and farther from the center of the system, it collided with one or more other planets. Researchers estimate one or more collisions occurred, each at a velocity exceeding 75 kilometers per second (270,000 kilometers per hour/167,000 miles per hour). Those collisions could have stripped away lighter elements like water and gases in the atmosphere, leaving only the dense rocky material behind.

It probably started out as a planet more similar to our own gas and ice giants but migrated closer to its star, smashing into other worlds along the way. By the time it got to its current location, it had gathered the strength of half a dozen planets its size. It’s dangerous out there, you never know when an entire world might absorb your own to make itself stronger.

Catch the complete first season of The Ark, streaming now on Peacock!