Surviving core of ill-fated Jupiter-like planet spotted near distant star

James Marshall
July 2, 2020

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.

The newly discovered exoplanet TOI 849 b offers the unique opportunity to peer inside the interior of a planet and learn about its composition. This utilises the Doppler effect to measure the mass of exoplanets by measuring their "wobble" - small movements towards and away from us that register as tiny shifts in the star's spectrum of light.

"This planet could have been a gas giant like Jupiter, which then lost its outer envelope through some violent evolution".

The study was published in the journal Nature.

Data taken by TESS, in the form of a star's light curve, or measurements of brightness, is first made available to the TESS science team, an worldwide, multi-institute group of researchers led by scientists at MIT.

The TESS observations revealed that TOI-849b lies extremely close to its host star, completing one lap every 18 hours.

Lead author Dr David Armstrong from the University of Warwick Department of Physics said: "While this is an unusually massive planet, it's a long way from the most massive we know. In other words, we don't see planets with this mass at these short orbital periods". The fact that it's in a odd location for its mass also helps - we don't see planets with this mass at these short orbital periods.

The planet's high density means that it primarily has to consist of iron, rock, and water, but only very little hydrogen and helium.

"We would expect a planet this massive to have accreted large quantities of hydrogen and helium when it formed, growing into something similar to Jupiter".

Located around a star much like our own approximately 730 light years away, the core, named TOI 849 b orbits so close to its host star that a year is a mere 18 hours and its surface temperature is around 1800K.

A combination of the absence of the expected gasses, the dense makeup of the alien world and its tight orbit led the researchers to conclude that TOI 849 b was nothing less than an exposed planetary core.

For the most part, astronomers focus their search for planets on the nearest, brightest stars that TESS has observed.

"I think one of the biggest clues is that we found the planet inside the "Hot Neptunian desert", which is this region of parameter space where we don't typically find planets", Dr Armstrong told BBC News.

"If this scenario is true, TOI-849 b is the only remnant planet core, and the largest gas giant core known to exist", says Huang. The first is that it was once similar to Jupiter but lost almost all of its outer gas through a variety of methods.

Tidal disruption, where the planet is ripped apart from orbiting too close to its star, or even a collision with another planet, might have played a role.

However, this would not have accounted for the entirety of a gas giant's atmosphere.

Alternatively, TOI 849 b could be a "failed" gas giant.

Scientists usually are not guaranteed whether or not the main dropped its environment in a collision or just under no circumstances developed one.

The discovery marks the first time a possible gas giant core has been spotted in space, allowing researchers to glance inside gas-giants like Saturn and Jupiter.

The core was discovered by NASA's Transiting Exoplanet Survey Satellite (TESS), which is created to observe a vast swathe of the night sky and look out for the tiny dips in starlight that occur as planets pass between the telescope and a distant star's surface. But astrophysicists on NASA's TESS mission have now found an exoplanet, TOI-849b, that appears to be 40 times more massive than Earth, yet just as dense. As it scans the sky, TESS monitors the light from the brightest, nearest stars, and scientists look for periodic dips in starlight that may signal that a planet is crossing in front of a star.

This could be because the primordial disk was disrupted in some way, leaving gaps that prevented a young TOI 849 b from grabbing material. These are, for example, submodels of accretion (growth of a planet's core) or of how planets interact gravitationally and influence each other, and of processes in the protoplanetary disks in which planets are formed.

Regardless of how it came to be, the discovery of TOI 849 b has given astronomers a rare chance to observe the exposed core of an alien world.

Other reports by Click Lancashire

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