Astronomers finally get to see what's inside a gas giant planet

Surviving core of Jupiter-like planet spotted near distant star

This orbital region is known as the "Neptunian desert", because Neptune-size worlds are very rare there.

It offers the unique opportunity to peer inside the interior of a planet and learn about its composition.

Neptune-sized planet may be remnant core of a much larger planet. "Although photoevaporation charges cannot account for the mass decline needed to minimize a Jupiter-like gas giant, they can take out a compact (a number of Earth masses) hydrogen and helium envelope on timescales of a number of billion years, implying that any remaining atmosphere on TOI-849b is most likely to be enriched by water or other volatiles from the planetary interior".

According to researchers from the University of Warwick's department of physics, the atmosphere could have been stripped away or it may have failed to form early on in the planet's life. That's when they came across TOI 849 b, a large object circling a sun-like star about 730 light-years away from Earth.

"We kept taking data and it turned out to still be a planet - just an exceptionally massive one for its size", Dr Armstrong explained.

TOI-849b has an equilibrium temperature of 1,527 degrees Celsius (2,781 degrees Fahrenheit) and an orbital period of only 18 hours, making it an "ultra-short-period" object.

TOI-849b has a radius smaller than Neptune's but an anomalously large mass of 39.1 times that of Earth, almost half the mass of Saturn.

TOI 849 b is also believed to be extremely dense and may "primarily 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".

Artist's impression showing a Neptune-sized planet orbiting a star located 730 light yearsfrom our solar system. REUTERS
Artist's impression showing a Neptune-sized planet orbiting a star located 730 light yearsfrom our solar system. REUTERS

"For such massive planets, gases like hydrogen and helium fall onto the forming planet very quickly, until the planet becomes something like Jupiter", said Armstrong, an astrophysicist at the University of Warwick.

Our best shot at seeing the core of a gas giant may be to spot a planet's core that has already been burned off by a nearby star.

"The planet could have been a gas giant before undergoing extreme mass loss via thermal self-disruption or giant planet collisions, or it could have avoided substantial gas accretion, perhaps through gap opening or late formation", the researchers write.

Although it's not clear how TOI 849 b lost its gaseous envelope, the astronomers have proposed two plausible scenarios. The University of Bern explains that this is the first time in history that an intact exposed core of a gas giant has been discovered orbiting around a star. Large-scale photoevaporation of the atmosphere could also play a role, but can't account for all the gas that has been lost.

Using the "Bern Model of Planet Formation and Evolution", the researchers devised to theories for TOI 849 b's existence and development into an exoplanet with an exposed core. "Once the core of the gas giant formed then something very unusual could have happened and it never formed a massive atmosphere as normally".

Astronomers have spotted the exposed core of a massive alien world, an unprecedented find that could shed considerable light on planet formation, evolution and diversity. "We have the opportunity to look at the core of a planet in a way that we can't do in our own solar system".

Now, a new planet discovered by NASA's Transiting Exoplanet Survey Satellite, TESS, seems to buck this trend. A dip in brightness indicates that a planet has passed in front of them.

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.

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