The discovery of a Neptune-sized exoplanet in orbit around a hot, bright giant star may help explain why such worlds are rare.
UC Berkeley researchers have discovered a Neptune-sized planet orbiting a massive, bright star. The gas giant, named HD 56414 b, is being stripped of its atmosphere by its host star.
This means that Neptune-sized planets orbiting bright, massive stars could be stripped of their outer layers of gas by intense stellar radiation. This process reduces these planets to nearly undetectable cores, which may explain why astronomers haven’t found more.
related: Exoplanets: Worlds Beyond the Solar System
“It’s one of the smallest planets we know of around these really massive stars,” UC Berkeley graduate student Steven Giacalone said in a statement. (opens in new tab)“In fact, it’s the hottest star we know of, and it has a planet smaller than Jupiter. This planet is interesting in the first place because these types of planets are really hard to find and we probably won’t be here on Earth. Find a lot of these planets.” For the foreseeable future. “
Despite being one of the smallest planets ever discovered around such a star, NASA’s TESS mission found HD 56414 b as it passed over its star’s surface, which is still 3.7 times the size of Earth. The Neptune-sized planet is so close to its star that it takes just 29 Earth days to orbit it.
In the 30 years since the first exoplanet was discovered in 1992, astronomers have discovered about 5,000 worlds around other stars. However, exoplanet discovery strongly favors planets around stars smaller than the sun — red dwarfs — or stars slightly larger than our average-sized star. About 99% of exoplanet discoveries fit this scenario.
Beyond that, most exoplanets discovered so far are Jupiter-sized worlds close to their parent stars.
Very few exoplanets are found around the largest stars or A-type stars, which also happen to be the brightest, hottest, and shortest-lived stellar bodies. And of these worlds, the smaller ones are even fewer.
That means less than 1 percent of the gas giants seen around massive stars are less massive than Jupiter.
This may be in part because Jupiter-sized worlds, with their greater gravitational influence, can be held in atmospheres more tightly than smaller, Neptune-sized planets, even though both worlds have a “fluffy” gas composition.
“There’s this balance between the Earth’s central mass and how much the atmosphere is expanding,” explains Giacalone. “For Jupiter or larger planets, the planet is massive enough to hold its expanding atmosphere through gravity. When you move down to a Neptune-sized planet, the atmosphere is still expanding, but the planets aren’t that big, so they’re more likely to lose atmosphere.”
This means that a Jupiter-sized world is unlikely to shrink to a nearly invisible core, even close to its parent star.
warm and hot neptune
HD 56414 b, what astronomers call “warm Neptune,” lies outside the region around the star where the planet would be violently stripped of its atmosphere. Its discovery could mean that in orbits closer to bright A-type stars, there are masses of unseen “hot Neptunes” detached from their outer layers and left behind as smoldering cores.
Astronomers have determined that there are fewer Neptune-sized planets orbiting less massive Sun-like stars than expected. The lack of detection of such worlds has been described as “hot Neptune deserts”, but it is uncertain whether hot Neptunes around A-type stars cannot be detected for the same reason, or because these planets are difficult to detect because their parent stars are more Bright.
“Determining whether the hot Neptune desert also extends to A-type stars provides insight into the importance of near-ultraviolet radiation in controlling atmospheric escape,” Courtney Dressing, an assistant professor of astronomy at UC Berkeley, said in the statement.
Solving this mystery and closing the gap in the exoplanet catalog may require more sensitive detection methods. Dressing and Giacalone will continue their search for Neptune-sized worlds around A-type stars and the fiery Neptune desert.
At the same time, the discovery of this warm Neptune could help astronomers better understand how the atmospheres of exoplanets evolve, where these planets form, and whether they move in or out as they evolve.
“This result is important for understanding the physics of atmospheric mass loss and for studying the formation and evolution of asteroids. It’s a big question about how planets maintain their atmospheres over time,” Dressing said. “When we look at a smaller planet, are we looking at the atmosphere that formed when it first formed from the accretion disk? Are we looking at the atmosphere expelled from the planet over time?”
Dressing went on to add that if astronomers can observe planets that receive different amounts of light from their stars — especially at different wavelengths of light — it could help model how planets maintain their atmospheres over time.
The study of A-type stars should allow for this, because planets closely orbiting them receive much more near-ultraviolet radiation than X-ray radiation or extreme ultraviolet radiation.
The researchers concluded that the planet’s proximity to a star in the system that is only 420 million years old — much younger than our sun’s 4.6 billion years old — means radiation is stripping its atmosphere.
HD 56414 b will still remain in its atmosphere for a billion years, long enough to see its parent star reach the end of its fusion fuel. This would cause the star to gravitationally collapse, triggering a supernova explosion that could leave a neutron star or black hole behind.
The team’s research is published in a paper in the August 12 edition astrophysical journal letters (opens in new tab)
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