A newly discovered planet is Jupiter’s diameter but eight times the mass, giving it twice the density of Earth, despite being mostly gas. These properties of this “super Jupiter” have not only puzzled astronomers, but may challenge current theories of planet formation.
tea exoplanetwhich lies about 310 light-years out solar system in the constellation Centaurus, orbits a sun-like one star and is only 15 million years old, making it a relative infant in cosmic terms and compared to our 4.6 billion year old planet. A team of astronomers has been able to measure both the diameter and mass of this gas giant – dubbed “SuperJupiter” because it is more massive than its namesake in the Solar System – making it the youngest planet of its kind to be known ever such measurements were made.
And those stats are weird. Explain how this planet, designated HD 114082 b, came to have eight times its mass crammed into a Jupiter-similar diameters may require updating planetary formation models, allowing 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 an age of only 15 million years,” says Olga Zakhozhay, astronomer at the Max Planck Institute for Astronomy in Germany and lead author of the new research said in a expression.
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The diameter and mass of HD 114082 b make it twice as dense Earth – amazing considering it is a gas giant composed mostly of hydrogen and helium, the lightest elements in the universe.
Orbiting its star at a distance half that between Earth and the Sun, the exoplanet completes an orbit every 110 Earth days, an orbit comparable to that of mercurythe planet closest to the sun.
A recipe for a strange Super Jupiter
There are two 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 mechanism of formation, the core accretion model, involves a protoplanet beginning life as a solid, rocky core that accumulates more and more material. Once this core reaches a critical mass, its gravitational pull pulls surrounding gas toward it, causing the core to accumulate hydrogen and helium in a runaway process that creates a giant planet.
The second mechanism, the disk instability model, involves gravitationally unstable and dense areas of the protoplanetary disk collapsing and growing to form a gas giant with no rocky core.
These formation models differ in the rate at which the accumulated gas cools, leading astronomers to describe planets as having a “hot” (core accretion) or “cold” (disk instability) start. Scientists currently prefer the hot start model, but the two approaches should result in observable differences and point scientists to the correct formation model.
For gas giants, that key feature is size: since hot gas occupies a larger volume than cold gas, smaller gas giants may have formed from a “cold” start, while larger gas giants like HD 114082 b are more likely to have formed from core accretion. The difference in size caused by the two possible origins should be particularly pronounced for younger worlds, becoming less and less measurable over hundreds of millions of years as the planet cools and the gas contracts.
Although hot-start is the commonly expected model, the density of HD 114082 b appears to contradict what astronomers would expect from a core accretion model, favoring the outsider, cold-start, or plate instability model instead. Some older exoplanets discovered by other teams of astronomers also favor this cold model, but the team behind the new research cautions against discarding models of hot-start planet formation just yet.
Alternative explanations for HD 114082 b’s small size and large mass, saving the critical mass model, include the idea that the exoplanet simply buried an extraordinarily large rocky core at its heart, or that astronomers don’t yet have an accurate picture of it, how fast gas inrushes a baby gas giant cools down.
“It’s far too early to give up the notion of a hot start,” Ralf Launhardt, an astronomer at the Max Planck Institute for Astronomy and a co-author of 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 discovered as part of the Radial Velocity Survey for Planets Around Young Stars (RVSPY) program conducted with the 2.2-metre 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 are using data collected by RVSPY to look for shifts in the light spectra of stars that indicate a “wobble” caused by an orbiting exoplanet. This technique, known as the radial velocity method, can also reveal a planet’s mass, but to measure Earth’s size, astronomers must watch as it crosses, or “traverses,” the face of its star, resulting in a tiny drop in light output.
This transit method can also help refine the exoplanet’s orbital period around its star, but is limited to planets that are actually crossing the front of their star as seen from Earth. Fortunately, HD 114082 b is just such a world, confirmed by the team using NASA’s exoplanet-hunting Transiting Exoplanet Survey Satellite (TESS).
“We already suspected a near-edge configuration of the planet’s orbit from a dust ring around HD 114082 discovered a few years ago,” Zakhozhay said in the statement. “Nevertheless, we were fortunate to find an observation in the TESS data with a nice transit light curve that improved our analysis.”
So far, HD 114082 b is one of only three giant planets younger than 30 million years that astronomers have determined both mass and size for. All of these planets appear to be incompatible with core accretion.
Although this is a very small data set, the team believes these planets are unlikely to be outliers and are indicative of a broader trend.
“Although more such planets are needed to confirm this trend, we believe theorists should start reassessing their calculations,” Zakhozhay said. “It is exciting how our observation results are incorporated into the theory of planet formation. They help improve our knowledge of how these giant planets grow and show us where the gaps in our understanding lie.”
tea team insights were published as a letter to the editor in Astronomy & Astrophysics magazine on Friday (November 25).
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