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

A newly found massive exoplanet has left astronomers deeply perplexed.

After taking measurements of a very young Jupiter-sized exoplanet called HD-114082b, scientists have discovered that its properties do not perfectly match either of the two popular models of gas giant planet formation.

Simply put, it’s 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 of Astronomy in Germany.

Orbiting a star called HD-114082 about 300 light-years away, the exoplanet has been the subject of an intense 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 about how planets form, a process that is not fully understood.

Two types of data are needed for a thorough characterization of an exoplanet, depending on the effect it has on its 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 the brightness of the star, this faint dimming can reveal the size of the exoplanet.

Radial velocity data, on the other hand, is a record of how much a star moves 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 radial velocity observations of HD-114082. Using the combined transit and radial velocity data, the researchers determined that HD-114082b has the same radius as jupiter – but it is 8 times the mass of Jupiter. This means that the exoplanet is about twice the density of Earth, and almost 10 times the density of Jupiter.

The size and mass of this young exoplanet means that it is highly unlikely to be a very large rocky planet; the upper limit for these is around 3 terrestrial radii i 25 land masses.

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

HD-114082b exceeds these parameters, which means it is a gas giant. But astronomers don’t know how it got that way.

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

Both ways are called “cold boot” or “warm boot”. In a cold start, the exoplanet is thought to form, pebble by pebble, from the debris of the disk orbiting the star.

The pieces are attracted, first electrostatically, then gravitationally. The more mass it gains, the faster it grows, until it is massive enough to trigger an accumulation of hydrogen and helium, the lightest elements in the Universe, resulting in a massive gaseous envelope around a rocky core.

Since gases lose heat as they fall toward the planet’s core and form an atmosphere, it’s considered the relatively cool option.

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

Exoplanets that experience a cold or hot start 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; their size and mass are more consistent with core accretion. But even so, it’s still too massive for its size. Either you have an unusually stuck core or something else is going on.

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

The exoplanet is one of three we know of that are 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 by three suggests that perhaps core accretion may be the more common of the two.

“Although more planets are needed to confirm this trend, we think theorists should start reevaluating their calculations.” Zakhozhai says.

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

The research was published in Astronomy and Astrophysics.