Exposed core of ancient gas giant discovered orbiting Sun-like star

Exposed core of ancient gas giant discovered orbiting Sun-like star
Artist's impression of the exposed core being bathed in radiation from its parent star
Artist's impression of the exposed core being bathed in radiation from its parent star
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Artist's impression of the exposed core being bathed in radiation from its parent star
Artist's impression of the exposed core being bathed in radiation from its parent star

Astronomers have discovered the intact, exposed core of a gas giant locked in a close orbit around a Sun-like star. The discovery will give astronomers a rare opportunity to probe the heart of an ancient alien world, and their findings could tell us more about what lies at the heart of the gas giants in our own solar system, including Jupiter and Saturn.

The core was discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS), which is designed 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.

TESS recorded one such fluctuation in a Sun-like star located some 730 light years from Earth. An analysis of the data revealed that the star was being orbited very closely by an object – now designated TOI 849 b – which was roughly the size of the planet Neptune in our own solar system.

According to the team, TOI 849 b is so close to its star that it completes an orbit once every 18 hours. Along with giving the strange object an incredibly short year, the tight orbit would bombard TOI 849 b with an overabundance of radiation from its parent star, resulting in a surface temperature of around 1,800 K.

The stellar companion was found to orbit in what is known as the Neptunian desert, a region of space surrounding a star where, based on current models, astronomers would not expect to find worlds larger than Neptune.

Following the initial discovery, the astronomers used the HARPS instrument mounted on the European Southern Observatory’s 3.6-m telescope located in the Chilean Atacama desert – to conduct follow up observations. HARPS is capable of accurately tracking minute wobbles in the motion of an exoplanet as it orbits a star. These wobbles are manifested as slight shifts in the planet’s light signature – which is called a spectrum – created as the object moves slightly closer to, and farther away from the watching telescope.

From the HARPS data, it was possible to calculate the mass of TOI 849 b. Once combined with measurements taken of the planet’s size, the researchers were able to estimate its density, and from that make a best guess of what that world is made of.

TOI 849 b was found to have a mass the equivalent to roughly 40 Earths, however it only had the radius 3.4 times that of our Blue Marble. Therefore, it had to be extremely dense, suggesting that it mostly comprises iron, rock and water, and very little of the relatively light elements hydrogen and helium.

This is strange, as an object like TOI 849 b would be expected to accrue large amounts of hydrogen and helium during the planetary formation process. Put simply, it should be a gas giant akin to Jupiter in our home solar system.

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.

The team then sought to determine the scenarios that could have created the unusual system. To this end the researchers fed their data into a scientific model known as the Bern Model of Planet Formation and Evolution. The Bern model takes into account a range of processes known to be pivotal in the development and evolution of planets, such as the behavior of accretion disks from which they are known to coalesce, and how young planetary body gravitationally interact with each other.

Two possible scenarios were put forth to explain the events leading to the discovery of the exposed core.

It is possible that the object was indeed once similar to Jupiter, and that it subsequently had its vast gaseous envelope depleted. Intense levels of stellar radiation could have stripped large amounts of material from the surface of the gas giant, blasting it into space.

However, this would not have accounted for the entirety of a gas giant’s atmosphere. There would also have had to have been a dramatic collision with another planetary body sometime in the distant past, or the planet could have been seriously disrupted upon passing too close to its parent star.

Alternatively, TOI 849 b may simply be a failed gas giant. In this scenario, the core of a gas giant successfully formed in the star’s accretion disk, but for one reason or another, it was unable to accrue the massive amounts of hydrogen and helium needed to become a Jupiter-like body.

This could be because the primordial disk was disrupted in some way, leaving gaps that prevented a young TOI 849 b from grabbing material. On the other hand the core could simply have formed when the disk was already exhausted, and there was simply not enough material to complete the creation process.

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. This in turn could grant insights into the formation of our own solar system, and what lies beneath the swirling hypnotic surfaces of Saturn and Jupiter.

The author’s of the paper don’t yet empirically know the composition of TOI 849 b. However, future observations may solve that mystery, as any gasses detected forming a thin atmosphere around the core would have to have emanated from within the tortured body. Therefore sniffing out the composition of atmospheric gasses would tell scientists what the core is made of.

The paper has been published in the journal Nature.

Source: University of Warwick

Bob Stuart
That's some pretty tough water!
That's an awful lot of speculation to assume it ever was a gas giant. An 18 day orbit would nearly put it in the corona of the star. Solar flares would literally burn it to a crisp leaving only a dense core.