A Strange Planet has been Found that’s Smaller than Neptune But 50% More Massive

Astronomers have found another strange exoplanet in a distant solar system. This one’s an oddball because its size is intermediate between Earth and Neptune, yet it’s 50% more massive than Neptune.

Astronomers have found what they call “puff planets” in other Solar Systems. Those are planets that are a few times more massive than Earth, but with radii much larger than Neptune’s. But this planet is the opposite of that: it’s much more massive than Neptune, but it also has a much smaller radius. Super-dense, not super-puffy.

This oddball planet is calling into question our understanding of how planets form.

Astronomers with the National Science Foundation NOIRLab studied the planet and presented their findings in a paper titled “The Habitable-zone Planet Finder Reveals A High Mass and a Low Obliquity for the Young Neptune K2-25b.” Lead author is Gudmundur Stefansson, a postdoctoral fellow at Princeton University. The paper will be published in The Astronomical Journal and is available at arxiv.org.

The Kepler mission found the planet in 2016. It orbits K2-25, an M-dwarf star in the Hyades star cluster. K2-25b orbits its star every 3.5 days. According to this study, it has a mass of about 24.5 Earth masses, and a radius of about 3.4 Earth radii. In the intro to their study, the authors write that “These properties are compatible with a rocky core enshrouded by a thin hydrogen-helium atmosphere (5% by mass).”

K2-25b orbits its host star K2-25 in the Hyades star cluster. Image Credit: NOIRLab/NSF/AURA/Digitized Sky Survey 2

The planet caught the interest of astronomers partly because it’s a “sub-Neptune” planet. A sub-Neptune can have either a mass larger than Neptune’s along with a smaller radius, like this one, or it can be less massive than Neptune, but with a larger radius. Either way, sub-Neptunes conflict with our models of planet formation. Understanding how these types of planets form is a critical question right now, on the frontier of exoplanet science.

“K2-25b is unusual,” said lead researcher Gudmundur Stefansson, a postdoctoral fellow at Princeton University. “The planet is dense for its size and age, in contrast to other young, sub-Neptune-sized planets that orbit close to their host star,” said Stefansson in a press release. “Usually these worlds are observed to have low densities — and some even have extended evaporating atmospheres. K2-25b, with the measurements in hand, seems to have a dense core, either rocky or water-rich, with a thin envelope.”

Astronomical models show that large planets form with a rocky core first. The initial mass for a core is modest, perhaps only 5 to 10 times more massive than the Earth itself. Then gas gathers around the core, creating a gaseous envelope that’s hundreds of times more massive than Earth. The gas giant Jupiter likely formed this way.

But a planets like K2-25b seem to show that our understanding is incomplete. It appears to have an enormously massive rocky core, with very little gaseous envelope. Its unusual properties pose a couple questions: How did it end up with such a massive rocky core? And, since it has such a massive core, how come it doesn’t have a large gaseous envelope?

An illustration of a protoplanetary disk. Planets coalesce out of the remaining material that the star formed out of. A rocky core forms, which then attracts a gaseous envelope. K2-25b is different somehow. It has a massive rocky core with a smaller gaseous envelope. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

While we don’t fully understand how K2-25b—and other planets like it—form, this planet is a sort of natural laboratory for studying them. “Given its known age and well characterized orbital parameters,” the authors write in their paper, “K2-25b is a benchmark system to study M-dwarf planet formation and subsequent dynamics, giving us further insights into the formation and migration mechanisms that produce other hot Neptune exoplanets.”

Given its large core, it should’ve acquired an enormous gaseous envelope. That fact that it didn’t gives rise to several possibilities. One of those possibilities, the authors suggest, is that the rocky core formed through mergers. “To explain its currently observed mass, we surmise that K2-25b could be the product of planet merging events of smaller planetary cores to produce a more massive planet.”

This figure from the study plots the mass radius plane for exoplanet K2-25b with other exoplanets, as well as the planets in our own Solar System. The red curve is plotted according to known models of planet formation, with a rocky core and a Hydrogen/Helium envelope. For a more detailed explanation, see the study. Image Credit: Stefansson et al, 2020.

That explanation also has the added benefit of accounting for the planet’s orbital eccentricity. They write: “Such a dynamical environment could have excited K2-25b into an eccentric orbit, and K2-25b could be in the process of migrating to a shorter period orbit through tidal interactions with the host star.”

Discoveries like this one are frequently the result of technological advances. Instruments like SPHERE, on the European Southern Observatory’s (ESO) Very Large Telescope (VLT), are responsible for many recent discoveries. But in this case, the piece of technology that made it possible was a $500 off-the-shelf diffuser. Lead researcher Gudmundur Stefansson developed a way of using Engineered Diffusers in his doctoral thesis.

An example of a 5 cm by 5 cm (2 inch by 2 inch) Engineered Diffuser.
Credit: Gudmundur Stefansson/RPC Photonics

An Engineered Diffuser spreads out the light from a star so that it covers more pixels on the camera. That allows the brightness of the star during the planet’s transit to be measured more accurately, which results in a higher-precision measurement of the size of the orbiting planet, among other parameters. “The innovative diffuser allowed us to better define the shape of the transit and thereby further constrain the size, density and composition of the planet,” said Jayadev Rajagopal, an astronomer at NOIRLab who was also involved in the study.

K2-25b is posing some important questions for astronomers. Answers to these questions will have to wait, but maybe not for too long. K2-25b is a prime candidate for follow-up observations with the James Webb Space Telescope. The Webb will have powerful onboard coronagraphs which will block out the light of exo-suns, making it easier to see the orbiting planets. It’ll also observe in infrared, something that it’ll excel at from its position at L2.

The Gemini South Telescope’s GHOST (Gemini High Resolution Optical SpecTrograph) also has K2-25b on its list of targets. It’s a spectrograph with a “wide simultaneous wavelength coverage at high observational efficiency” according to the website. It’ll be very effective at observing the atmosphere of planets like this one.

More observations with GHOST and the JWST may not answer all the questions that K2-15b poses about planet formation. But they will expand the boundaries of what we do and don’t know about it. In the mean-time, maybe more astronomers will come up with low-budget ways of addressing them.

The diffuser that made this study so effective was used with the WIYN 0.9-meter Telescope at Kitt Peak National Observatory (KPNO). The 3.5-meter telescope at Apache Point Observatory (APO) in New Mexico was also part of the study. The National Optical-Infrared Astronomy Research Laboratory (NOIRLab) includes the Gemini Observatory, the Kitt Peak Obsevatory, as well as several other facilities and the upcoming Vera C. Rubin Observatory.


Evan Gough

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