Exposed Planetary Core Discovered 730 Light Years Away

An artist’s impression showing a Neptune-sized planet in the Neptunian Desert. It is extremely rare to find an object of this size and density so close to its star. (University of Warwick/Mark Garlick)

(CN) — Scientists believe they have discovered the remnant core of a gas giant about 730 light years away, providing astronomers a rare look into the interior of a planet.

“It’s the first time that we’ve been sure that we’re seeing a core, period,” said David Armstrong, a physicist at the University of Warwick.

His research, produced in tandem with 94 other scientists across the globe, was published Wednesday in the journal Nature.

The planet — designated TOI-849b — was first seen in September 2018 by NASA’s Transiting Exoplanet Survey Satellite, a telescope launched that April as a successor to the 2009 Kepler mission. The satellite searches for exoplanets, planets outside the solar system, by observing stars for a dip in brightness that suggests a planet has passed in front of it.

As NASA’s satellite drifted on and lost sight of TOI-849b, ground-based telescopes in Chile followed up with their own imagery. They used the Doppler effect to measure the “wobble” of exoplanets, or the way they move toward and away from us, causing tiny shifts in the star’s light spectrum that help scientists indicate the planet’s mass.

Armstrong and his colleagues found that TOI-849b lacks the characteristic thick envelope of hydrogen and helium composition of gas giants such as Jupiter and Saturn. NASA’s satellite had discovered a former gas giant.

So why would TOI-849b lack that massive gaseous envelope?

“If we take a planet the size of Jupiter and then try and remove all of its atmosphere to get something like what we see here, that takes a really dramatic event — like planets colliding towards the end of their formation would blow off their atmospheres, and that might explain why we see such a massive core left, because it could combine the cores of the two colliding planets,” said Armstrong, the study’s lead author.

He added, “Another option is if the planet passes really close to its host star, that could cause tidal forces between the planet and the star system, ripping away the atmosphere and leaving the core behind.

TOI-849b was found in what astronomers call the Neptunian Desert, a region near stars known for its hostility to planets of Neptune’s mass or larger.

“The hot Neptunian Desert is this region [around a star] … where we just don’t see planets appearing, almost at all. For various reasons, that region of space is just very empty, and this planet sits right in the middle of it,” Armstrong said.

The planet is so close to its host star that it completes a year’s revolution in less than 18.5 hours, and its surface temperature measures about 1,800 degrees Kelvin, or 2,780 degrees Fahrenheit. The planetary core weighs nearly 39 times the Earth’s mass, its diameter is about 3.5 times our planet’s and is almost just as dense.

“For a gas giant core, this is very massive,” Armstrong said. “It’s actually quite a lot more massive than we expect the core of Jupiter to be.”

TOI-849b could also be a “failed” gas giant, one that never formed an atmosphere to begin with. The disc of dust that formed the planet might have had a gap in it, or the planet may have formed late and ran out of disc material to form an atmosphere with.

Little is known about giant planets’ interiors because they are so difficult to observe, so TOI-849b could be the key to future discoveries about the properties and composition of planetary cores.

“In a broad sense, this [research] is telling us that it’s possible to find cores like this by looking at planets in this kind of region, and if we can find more and build up a population of them, that really gives us a broader picture of how planet formations can occur,” Armstrong said. “That starts to let us get a better context for how our own solar system formed.”

Additionally, observations like these provide a concrete test for scientists’ theories about planetary formation, like a hurdle for models to clear.

“This certainly gives us a benchmark, extreme case that future models are going to have to be able to deal with,” Armstrong said. “These extreme cases have you really test out your theories. Something has to happen to make them, they can’t be impossible.”

Armstrong notes that future telescopic research may bring astronomers and physicists closer to unlocking the secrets of planets’ interior lives.

“We’re actually trying to get some telescopes going to work out whether the planet’s orbit aligned with the stellar spin — the spin axis of the star — which will tell us a bit about how it evolved,” Armstrong said. “Longer term, we really want to measure any constituents in the atmosphere of this exposed core, because all the atmosphere left and what’s in it might be very interesting. But it’s a very hard observation to make, so we’ll need next-generation telescopes for that.”

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