A Neptune-sized Exoplanet is Denser Than Steel. The Result of a Catastrophic Collision?

There’s an odd exoplanet out there posing a challenge to planetary scientists. It’s a hot Neptune denser than steel. The big question is: how did it form?

The planet is TOI-1538b and it orbits its parent dwarf star every 1.24 days. It’s classified by planetary scientists as a world in the “Hot Neptune Desert”. That means there aren’t as many of these close-in hot Neptunes as scientists expect. There are a few others like it, although not as dense.

The stats on this world are impressive. It has the equivalent of ~75 Earth masses and is about 3.5 Earth radii. At a density of ~9.7 gm/cm3, that implies the interior contains a lot more rocky stuff than expected. (For reference, steel can be as much as 8.0 gm/cm3.) And, that makes this place a puzzling find because its evolution doesn’t seem to fit conventional planetary formation theories.

Could Massive Collisions Be the Answer?

A team of scientists led by Luca Naponiello of Rome Tor Vergata suspected that multiple catastrophic planetary collisions shaped TOI-1538b. Those impacts removed lighter atmospheric gases and water, leaving behind a rocky core. It’s not a surprising conclusion, since planetary formation involves lots of smaller worlds smacking together to make bigger ones. So, why not big ones slamming into each other?

Senior Research Associate and study co-author Dr Phil Carter from the University of Bristol’s School of Physics explained the idea. “We have strong evidence for highly energetic collisions between planetary bodies in our solar system, such as the existence of Earth’s Moon, and good evidence from a small number of exoplanets,” he said. “We know that there is a huge diversity of planets in exoplanetary systems; many have no analog in our solar system but often have masses and compositions between that of the rocky planets and Neptune/Uranus (the ice giants).”

Collisions in Exoplanet Formation

Our own solar system provides a good model for the formation of exoplanet systems. Some 4.5 billion years ago, the proto-Sun began coalescing in a cloud of gas and dust. That nebula was rich with heavier elements useful for planetary formation. Smaller particles—planetesimals—slammed together in the resulting protoplanetary disk to make larger and larger bodies. The result was four small rocky bodies plus four gas- and ice-rich giant worlds. In addition, the solar system has dwarf planets, comets, asteroids, and moons.

Artist's conception of a solar system in formation. It's likely that exoplanet formation around other stars proceeded similarly. Credit: NASA/FUSE/Lynette Cook
Artist’s conception of a solar system in formation. It’s likely that exoplanet formation around other stars proceeded similarly. Credit: NASA/FUSE/Lynette Cook

Bombardment of these infant worlds continued, scarring some with craters (like Mercury), and flipping at least one (Uranus) on its side. Planetary scientists look to that formation history to understand how similar processes played out around other stars. Spacecraft such as the Kepler and TESS missions discovered more than 5,000 candidate worlds. Astronomers suspect the galaxy teems with millions of planets. Most systems appear to have similar collections of exoplanets to ours, although not always in the sizes and masses that match our own.

Collisions remain an important part of how this exoplanet and other worlds formed. “Our contribution to the study [of TOI-1853b] was to model extreme giant impacts that could potentially remove the lighter atmosphere and water/ice from the original larger planet in order to produce the extreme density measured,” said Carter. If those happen frequently, that opens up new avenues of study for planetary formation specialists.

Modeling a Steely Exoworld

To understand its formation history, a science team led by essentially modeled extreme giant impacts that could strip away atmospheric elements. They found that the proto-Neptune would have once been a very wet exoplanet. In order to lose all that material, an impactor had to slam into it at a speed of more than 75 meters per second. Given those conditions, they could model a planet very similar to TOI-1853b. According to team member Zoë Leinhardt, the type of planetary impact that created this exoplanet wasn’t something they’d thought about. “We had not previously investigated such extreme giant impacts as they are not something we had expected. There is much work to be done to improve the material models that underlie our simulations, and to extend the range of extreme giant impacts modeled,” she said.

A simulation of an impact that might have created TOI-1538b. Courtesy: Jingyao Dou.

The next step is to do follow-up observations of the planet. It’s important to find out what’s left of its atmosphere and the composition of its gases. With a “real-world” example of what the planetary scientists modeled, it seems likely others could exist. TOI-1853b provides new evidence for the prevalence of giant impacts in the formation of planets throughout the galaxy. This discovery helps to connect theories for planet formation based on the solar system to the formation of exoplanets.

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