A newly weighed exoplanet has left astronomers deeply perplexed.
After taking measurements on a very young, Jupiter-sized exoplanet named HD-114082b, scientists found that its properties don’t exactly match either of the two popular models of gas giant planet formation.
In short, it is much 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,” says astrophysicist Olga Zakhozhay of 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 intense data collection campaign. At just 15 million years old, HD-114082b is one of the youngest exoplanets ever discovered, and understanding its properties could provide clues to how planets form – a process that isn’t fully understood. .
Two types of data are needed for a full characterization of an exoplanet, based on the effect it has on its host star. Transit data is a record of how a star’s light fades as an orbiting exoplanet passes in front of it. If we know how bright the star is, this faint dimming may reveal the size of the exoplanet.
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 tug. 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 had the same radius as Jupiter, but 8 times the mass of Jupiter. This means that the exoplanet has about 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 very large rocky planet; the upper limit for these is about 3 Earth radii and 25 Earth masses.
There is also a very small density range in rocky exoplanets. Above this range, the body becomes denser and the planet’s gravity begins to retain a significant atmosphere of hydrogen and helium.
HD-114082b greatly exceeds these parameters, which means it is a gas giant. But astronomers just don’t know how it happened.
“We believe that giant planets can form in two possible ways,” says MPIA astronomer Ralf Launhardt. “Both occur inside a protoplanetary disk of gas and dust distributed around a central young star.”
Both methods are called “cold start” or “warm start”. During a cold start, the exoplanet is thought to form, pebble by pebble, from debris in the disk orbiting the star.
The coins are attracted, first electrostatically, then gravitationally. The more mass it gains, the faster it expands, until it is massive enough to trigger a galloping accretion of hydrogen and helium, the lightest elements in the Universe, resulting in a gaseous envelope massive around a rocky core.
Since gases lose heat as they fall towards the planet’s core and form an atmosphere, this is considered the relatively cool option.
A hot start is also known as disk instability, and is believed to occur when a swirling region of instability in the disk collapses directly on itself under the effect of gravity. . The resulting body is a fully formed exoplanet that lacks a rocky core, where gases retain most of their heat.
Exoplanets experiencing a cold or warm start are expected to cool at different rates, producing distinct features that we should be able to observe.
Properties of HD-114082b do not match hot-start model, researchers say; its size and mass are more consistent with core accretion. But even then, it’s still too massive for its size. Either he has an unusually chonky core or something else is going on.
“It’s far too early to abandon the notion of hot start,” says Launhardt. “All we can say is that we still don’t fully understand how giant planets form.”
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 disk instability model.
Obviously, three is a very small sample size, but three for three suggests that base accretion might be the more common of the two.
“Although more such planets are needed to confirm this trend, we think theorists should start re-evaluating their calculations,” Zakhozhay said.
“It’s exciting to see how our observational results feed into the theory of planet formation. They help improve our knowledge of the growth of these giant planets and tell us where the gaps in our understanding lie.”
The research has been published in Astronomy & Astrophysics.
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