A gas giant exoplanet has been discovered with twice the density of Earth: ScienceAlert

A gas giant exoplanet has been discovered with twice the density of Earth: ScienceAlert

A newly weighted exoplanet has left astronomers deeply puzzled.

After taking measurements of a very young Jupiter-sized exoplanet called HD-114082b, scientists have found that its properties don’t quite match either of the two popular models of gas giant planet formation.

Simply put, it’s way too heavy for its age.

“Compared to currently accepted models, HD-114082b is about two to three times too dense for a young gas giant only 15 million years old,” explains astrophysicist Olga Zakhozhay from the Max Planck Institute for Astronomy in Germany.

The exoplanet orbits a star called HD-114082 at a distance of about 300 light-years and has been the subject of an intensive data-gathering campaign. At just 15 million years old, HD-114082b is one of the youngest exoplanets ever found, and understanding its properties could provide clues to how planets form — a process that is not fully understood.

Two types of data are needed for a comprehensive characterization of an exoplanet, based on the effect it has on its parent star. Transit data is a record of how a star’s light dims when an exoplanet in orbit passes in front of it. Knowing how bright the star is, that faint dimming could reveal the exoplanet’s size.

Radial velocity data, on the other hand, is a record of how much a star wobbles in place in response to the exoplanet’s gravity. Knowing the mass of the star, the amplitude of its oscillation can give us the mass of the exoplanet.

For nearly four years, the researchers collected radial velocity observations from HD-114082. Using the combined transit and radial velocity data, the researchers determined that HD-114082b has the same radius as Jupiter – but is 8 times the mass of Jupiter. That means the exoplanet is about twice the density of Earth, and nearly 10 times the density of Jupiter.

The size and mass of this young exoplanet make it highly unlikely that it will be a super-sized rocky planet; the upper limit for that is round 3 earth rays and 25 Earth masses.

There is also a very small density range in rocky exoplanets. Above this range, the body gets closerand the planet’s gravity begins to trap a significant atmosphere of hydrogen and helium.

HD-114082b greatly exceeds those parameters, meaning it is a gas giant. But astronomers just don’t know how it happened.

“We think giant planets could form in two possible ways,” says astronomer Ralf Launhardt from MPIA. “Both occur in a protoplanetary disk of gas and dust scattered around a young central star.”

The two ways are called a ‘cold start’ or a ‘warm start’. The exoplanet is thought to form pebble by pebble from debris in the disk orbiting the star upon a cold start.

The pieces are attracted, first electrostatically, then gravity. The more mass it gains, the faster it grows, until it is massive enough to cause runaway accretion of hydrogen and helium, the lightest elements in the universe, resulting in a huge gaseous envelope around a rocky core.

Since the gases lose heat as they fall to the planet’s core and form an atmosphere, this is seen as the relatively cool option.

A hot start is also known as disk instability and is thought to occur when a swirling region of instability in the disk collapses directly on itself under the influence of gravity. The resulting body is a fully formed exoplanet that lacks a rocky core, where the gases retain more of their heat.

Exoplanets experiencing a cold start or a hot start must cool at different rates, producing different features that we should be able to observe.

HD-114082b’s properties don’t match the hotstart model, the researchers say; its size and mass are more consistent with nuclear accretion. But even then, it’s still just too massive for its size. Either it has an unusually lumpy core, or something else is going on.

“It’s way too early to let go of the idea of ​​a hot start,” Launhardt says. “All we can say is that we still don’t really understand the formation of giant planets.”

The exoplanet is one of three known to be less than 30 million years old and for which astronomers have obtained radius and mass measurements. So far, all three seem inconsistent with the disk instability model.

Obviously, three is a very small sample size, but three for three suggests that perhaps core accretion is the more common of the two.

“While more such planets are needed to confirm this trend, we believe theorists should reevaluate their calculations,” says Zakhozhay.

“It’s exciting how our observational results feed back into planet formation theory. They help improve our understanding of how these giant planets grow and tell us where the gaps in our understanding lie.”

The research has been published in Astronomy & Astrophysics.

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