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Gas giant exoplanet with twice the density of Earth discovered: ScienceAlert

Gas giant exoplanet with twice the density of Earth discovered: ScienceAlert

A recently weighed exoplanet has left astronomers deeply puzzled.

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

Simply put, he is too heavy for his 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 Zahozai from the Max Planck Institute for Astronomy in Germany.

Orbiting a star named HD-114082 about 300 light-years away, the exoplanet 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 discovered, and understanding its properties could provide clues about how planets form, a process that is not fully understood.

Two types of data are needed to comprehensively characterize an exoplanet based on the effect it has on the host star. Transit data is a record of how a star’s light dims when an orbiting exoplanet passes in front of it. If we know how bright the star is, this faint dimming can reveal the size of the exoplanet.

The radial velocity data, on the other hand, is a record of how much a star wobbles in place in response to the exoplanet’s gravitational pull. If we know the mass of the star, then the amplitude of its oscillation can give us the mass of the exoplanet.

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

The size and mass of this young exoplanet means it is highly unlikely to be a super-large rocky planet; the upper limit for them is approx 3 Earth radii and 25 Earth masses.

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

HD-114082b is in much more than these parameters, meaning it is a gas giant. But astronomers just don’t know how it happened.

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

The two ways are called “cold start” or “hot start”. In a cold start, the exoplanet is thought to form, pebble by pebble, from debris in the disk orbiting the star.

The particles are attracted, first electrostatically, then gravitationally. The more mass it gains, the faster it grows until it becomes massive enough to cause a rapid accretion of hydrogen and helium, the lightest elements in the universe, resulting in a massive gas shell around a rocky core.

Given that gases lose heat as they fall toward the planet’s core and form an atmosphere, this is seen as a relatively cool option.

A hot start is also known as a disc instability and is thought to occur when a rotating region of instability in the disc directly collapses in on itself under the influence of gravity. The resulting body is a fully formed exoplanet that does not have a rocky core where gases retain most of their heat.

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

HD-114082b’s properties don’t fit the hot-start model, the researchers say; its size and mass are more consistent with core accretion. But even then, it’s still too massive for its size. Either there is an abnormally broken core or something else is going on.

“It’s too early to abandon the idea of ​​a hot start,” Launhard says. “All we can say is that we still don’t understand the formation of the giant planets very well.”

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

Obviously three is a very small sample size, but three for three suggests that perhaps the underlying hoarding may be the more common of the two.

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

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

The study was published in Astronomy and astrophysics.

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