The Discovery of a Hot Neptune that Shouldn’t Exist

1800 light-years away, an unlikely survivor orbits an aged star. This rare planet is called a hot Neptune, and it’s one of only a small handful of hot Neptunes astronomers have found. Hot Neptunes are so close to their stars that the overpowering stellar radiation should’ve stripped away their atmospheres, leaving only a planetary core behind.

But this planet held onto its atmosphere somehow.

The more exoplanets we find, the better we understand the exoplanet population. With over 5,300 confirmed exoplanets, scientists are getting a handle on the makeup of the exoplanet population. On their Exoplanet Discovery Dashboard, NASA groups exoplanets into these categories: Neptune-like, Gas Giant, Super-Earth, and Terrestrial. Other classifications are used in space science, too, like hot Jupiter. But the hot Neptune tag isn’t used much because there aren’t many of them.

A hot Neptune is a gaseous planet that is extremely close to its star, just as a hot Jupiter is. When a gas planet gets too close to its star, the star can strip the gaseous atmosphere away. Hot Jupiters are so massive that they can hang on to their atmospheres with some success. But Neptune-size planets are much less massive than Jupiter-size planets, and really, hot Neptunes just shouldn’t exist. They’re not massive enough to hold onto their atmospheres in the face of all the stellar radiation. They defy the odds.

“The discovery of a low-density hot Neptune orbiting an evolved star demonstrates that the atmospheres of these planets are more resilient than previously thought.”

The first hot Neptune astronomers found is Gliese 436 b. Astronomers found it in 2004, five years before the Kepler mission changed exoplanet science forever. Researchers are still puzzling over Gliese 436 b and trying to understand how it held onto its atmosphere for this long. A 2015 paper concluded that the planet is losing mass and leaving a trail of hydrogen behind it as the star strips away its atmosphere.

Artist’s impression of JG436b, a hot Neptune located about 33 light years from Earth. The planet is still a puzzle, as are all hot Neptunes. Credit: Courtesy Space Telescope Science Institute

In new research, astronomers from the USA and Australia presented their discovery of another hot Neptune. The paper is “An unlikely survivor: a low-density hot Neptune orbiting a red giant star.” The lead author is Samuel Grunblatt from the Department of Physics and Astronomy at Johns Hopkins University.

This hot Neptune is so close to its star that it completes an orbit in only 4.2 days. The planet, named TIC 365102760 b, is not very dense. Even though it’s about half the radius of Jupiter, its density is only 0.06 that of Jupiter’s. With a density that low, the planet shouldn’t be hanging onto its atmosphere.

The star is ancient, a red giant about 7.2 billion years old, and is 1.2 times as massive as the Sun. Its temperature is 4700 Kelvin (4400 C; 8,000 F.) Considering all these factors, TIC 365102760 b should be nothing but a planetary core by now. “The old age and high equilibrium temperature yet remarkably low density of this planet suggests that its gaseous envelope should have been stripped by high-energy stellar irradiation billions of years ago,” the authors write.

This planet is the rarest of the rare. There is only a small handful of Neptune-size planets orbiting a post-main sequence star, and it’s the only hot Neptune orbiting this type of star. Outliers like these are important because they can define Nature’s limits and help scientists build better models.

Current models can’t explain TIC 365102760 b and instead show that there should be nothing left by now but a core. “Thus, assuming that the planet did not experience migration or inflation after a system age of 20 Myr, most or all of the planet’s atmosphere should have been stripped over its lifetime,” the authors write.

How was it able to hold onto its mass for so long while being so close to its star?

Atmospheric stripping is a well-known phenomenon. If a planet is too close to its star, or if it lacks a protective magnetic shield, powerful radiation can strip the planet’s atmosphere away. This artist’s rendering shows a solar storm hitting Mars and stripping ions from the planet’s upper atmosphere. Credits: NASA/GSFC

The authors say that the explanation could lie in incorrect models of stellar flux. If the star isn’t as powerful as thought, then that could explain how the planet has held onto its atmosphere for so long. “First, the stellar flux in XUV may be significantly lower or absorbed less efficiently than existing models predict, preventing severe atmospheric erosion even if the planet has not changed its orbit or radius since formation.”

Migration could explain the planet, too, but only if it migrated during the star’s main sequence lifetime. “Second, the planet may have migrated to its current orbit during the main sequence lifetime of its host star from a previous larger orbit, avoiding the highest intensity of XUV irradiation from its host star,” they explain.

But there are some problems with that explanation. Star-planet and planet-planet interactions could’ve caused the migration, but there’s no indication of another large planet relatively near TIC 365102760 b. Interactions between a planet and a star can trigger migration, but in those cases, the planet has a highly-eccentric orbit. “Furthermore, TIC 365102760 b does not appear to have a high-eccentricity orbit, suggesting that migration due to star-planet interaction is also unlikely or not very recent in the system’s history,” the authors write.

The researchers suggest a third possibility to explain the hot Neptune. It could’ve been significantly smaller in the past, “… limiting the instantaneous rate of mass loss on the main sequence.” Planets are known to inflate as their stars leave the main sequence due to an increase in received radiation. But at this level of detail, much of the potential explanation comes down to different models, and there’s simply not enough clarity to attach any certainty to the explanation.

Gaseous planets are known to inflate when their stars leave the main sequence and irradiate their planets more powerfully. This illustration shows the exoplanet K2-132b. The upper left shows the planet during the star’s main sequence lifetime, and the lower left panel shows how the planet expanded when the star left the main sequence. Image Credit: Karen Teramura, University of Hawaii Institute for Astronomy.

The researchers are shying away from the late-stage migration explanation. “Current observational evidence for both late-stage inflation and/or weak photoevaporation is stronger than evidence for late-stage migration in this system,” they explain. They think that a weaker level of XUV radiation from the star alongside late-stage inflation is the best explanation for this hot Neptune’s persistence.

But even without a clear explanation for TIC 365102760 b’s resilience, the planet and this research are telling planetary scientists some important things.

“The discovery of a low-density hot Neptune orbiting an evolved star demonstrates that the atmospheres of these planets are more resilient than previously thought,” the authors write. The planet’s existence also shows that planets smaller than Jupiter can become inflated as their stars evolve out of the main sequence. This has implications for our understanding of how Neptune-size planets form and evolve and implications for how scientists interpret the exoplanet population.

This figure from the research shows the planetary radius on the y-axis and atmospheric loss divided by the planetary mass on the x-axis. TIC 365102760 b is denoted by the star symbol. The exoplanet’s atmospheric mass is predicted to be smaller than the total mass expected to have been lost. It’s a puzzle how the planet has retained an atmosphere given its radius and orbit near its star. Image Credit: Grunblatt et al. 2023.

Since the star and planet are so closely intertwined, these results can tell us something about planet composition and stellar activity, too. But that still needs to be untangled. Finding more of these rare planets is the obvious path forward, but they don’t often appear in general searches. “Focused searches for these evolved systems are necessary as these planets are missed by general searches for transiting planets,” the authors write.

As for TIC 365102760 b’s specific case, follow-up observations could more tightly constrain the planet’s characteristics and help explain its existence. Ground-based and space-based spectroscopic studies could reveal atmospheric outflows from the planet more clearly and could shed light on the lifetime of its atmosphere as well as the atmosphere’s composition.

“Constraining the balance between planet atmospheric inflation and mass loss will help reveal the evolution of planetary atmospheres over time, clarifying planet demographic features such as the hot Neptune desert,” the authors conclude.