A newly discovered planet has the diameter of Jupiter but eight times its mass, giving it twice the density of Earth despite being composed mostly of gas. Not only have these characteristics of this “super Jupiter” left astronomers confused, but they could also challenge current theories about planet formation.

The exoplanet, which lies around 310 light-years outside the solar system in the constellation Centaurus, orbits a sun-like star and is just 15 million years old, making it a relative infant in cosmic terms and when compared to our 4.6 billion-year-old planet. A team of astronomers was able to measure both the diameter and mass of this gas giant — dubbed a “super Jupiter” because it is more massive than its solar system namesake — making it the youngest planet of this kind for which such measurements have ever been made.

And those statistics are strange. Explaining how this planet, designated HD 114082 b, came to have eight times the mass crammed into a Jupiter-like diameter may require an update to planetary formation models that allows gas giants to possess unusually large solid planetary cores.

“Compared to currently accepted models, HD 114082 b is about two to three times too dense for a young gas giant with only 15 million years of age,” Olga Zakhozhay, an astronomer at the Max Planck Institute for Astronomy in Germany and lead author of the new research, said in a statement. 

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HD 114082 b’s diameter and mass give it a density that is twice that of Earth — astounding given that it’s a gas giant composed mostly of hydrogen and helium gas, the universe’s lightest elements.

The exoplanet circles its star at a distance that is half that between Earth and the sun, completing an orbit every 110 Earth days, an orbit comparable to that of Mercury, the closest planet to the sun. 

A recipe for a weird super-Jupiter 

There are two possible ways a gas giant like HD 114082 b could form, both of which occur in the protoplanetary disk, a disk of gas and dust that collapses to form planets. 

The first formation mechanism, the core accretion model, involves a protoplanet starting life as a solid, rocky core accumulating more and more material. Once this core attains a critical mass, its gravitational influence drags surrounding gas to it, resulting in the core accreting hydrogen and helium in a runaway process that births a giant planet.

The second mechanism, the disk instability model, involves gravitationally unstable and dense patches of the protoplanetary disk collapsing and growing to form a gas giant lacking a rocky core.

These formation models differ in the rate at which the gas accumulated cools down, leading astronomers to describe planets as getting a “hot” (core accretion) or “cold” (disk instability) start. Scientists currently favor the hot-start model, but the two approaches should lead to observable differences, thus pointing scientists toward the right formation model. 

In gas giants, that key characteristic is size: Because hot gas occupies a larger volume than cold gas, smaller gas giants might have formed from a “cold” start, whereas larger gas giants like HD 114082 b more likely formed by core accretion. The difference in size caused by the two potential origins should be particularly pronounced among younger worlds, becoming less and less measurable over hundreds of millions of years as the planet cools and the gas contracts. 

Despite hot-start being the commonly expected model, HD 114082 b’s density seems to defy what astronomers would expect for a core accretion model, favoring instead the underdog, the cold start or disk instability model. Some older exoplanets discovered by other teams of astronomers also favor this cold model, but the team behind the new research warns not to scrap hot start planet formation models just yet. 

Alternative explanations for HD 114082 b’s small size and big mass that rescue the critical mass model include the idea that the exoplanet simply has an exceptionally large rocky core buried at its heart or that astronomers don’t yet have an accurate picture of how rapidly gas in an infant gas giant cools.

“It’s much too early to abandon the notion of a hot start,” Ralf Launhardt, an astronomer at Max Planck Institute for Astronomy and co-author on the new research, said in the statement. “All we can say is that we still don’t understand the formation of giant planets very well.”

Star’s ‘wobble’ reveals exoplanet HD 114082 b  

HD 114082 b was spotted as part of the Radial Velocity Survey for Planets Around Young Stars (RVSPY) program, operated using the 2.2-meter telescope at the European Southern Observatory’s (ESO) La Silla site in Chile. The program aims to uncover the population of hot, warm and cold giant planets around young stars.

Astronomers use data collected by RVSPY to hunt for shifts in the spectra of light from stars that indicate a “wobble” caused by an orbiting exoplanet. Known as the radial velocity method, this technique can also reveal a planet’s mass, but to measure the world’s size, astronomers must observe it as it crosses or “transits” the face of its star, causing a tiny dip in light output.

This transit method can also help refine the orbital period of the exoplanet around its star, but it’s limited to planets that actually cross the face of their star as seen from Earth. Fortunately, HD 114082 b is just such a world, which the team confirmed with NASA’s exoplanet-hunting Transiting Exoplanet Survey Satellite (TESS).

“We already suspected a nearly edge-on configuration of the planetary orbit from a ring of dust around HD 114082 discovered several years ago,” Zakhozhay said in the statement. “Still, we felt lucky to find an observation in the TESS data with a beautiful transit light curve that improved our analysis.”

Thus far, HD 114082 b is one of only three giant planets younger than 30 million years for which astronomers have determined both masses and sizes. All of these planets seem to be inconsistent with the core accretion. 

Even though this is a very small data set, the team believes these planets are unlikely to be outliers and are indicative of a wider trend. 

“While more such planets are needed to confirm this trend, we believe that theorists should begin re-evaluating their calculations,” Zakhozhay said. “It’s exciting how our observational results feed back into planet formation theory. They help improve our knowledge about how these giant planets grow and tell us where the gaps of our understanding lie.”

The team’s findings were published Friday (Nov. 25) as a Letter to the Editor in the journal Astronomy & Astrophysics.

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