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Strange Sub-Neptune Planet Shouldn’t Exist

Artist drawing of purple planet beside red star
An artist’s impression of K2-25b orbiting around its M-dwarf star. NOIRLab/NSF/AURA/J. Pollard

Planet K2-25b is located in the Hyades star cluster and makes a full orbit of its M-dwarf star about every 3.5 days. None of that is unusual. In fact, M-dwarf stars are the most plentiful in the galaxy and planets the size of K2-25b, which are known as Sub-Neptune planets because they are sized between Earth and Neptune, are often found orbiting around them. 

Unusual Composition

Typically planets of this size begin with an icy, rocky core that’s about 5-10 times the mass of Earth. At that point, the accumulation of solid material stops, and the gravity of the core proceeds to draw in a gaseous covering that is hundreds of times the mass of our own planet. That’s how the gas giants in our solar system formed.

But in the case of K2-25b, the planet has a giant core that’s about 25 times that of Earth’s and only a relatively thin covering of gas. So even though it is smaller than Neptune, its mass is about 1.5 bigger. While scientists aren’t quite sure how the planet formed in this odd way, they do have some theories.

“One possible way to create the dense planet is that K2-25b  could be the result of a merger of planetary cores, where original planets or planetary cores that had already formed collided and merged to form a single more massive planet,” lead researcher Gudmundur Stefansson of Princeton University told Mindbounce. “These planet-planet colliding events heat the resulting merged core, and can cause a substantial fraction of gases to be lost/stripped away.”

Stefansson also told us that the planet might have formed at a time when there simply wasn’t enough gas in the protoplanetary disk when the planet formed, so it grabbed more rocks and ice instead. 

Planet-Hunting Tech

The new findings about the planet, which was first spotted by NASA’s now-decommissioned Kepler satellite, were accomplished through the use of an Engineered Diffuser, which spreads the light from a star over more pixels on a camera. It was attached to the telescope at the Apache Point Observatory in New Mexico. Also adding information to the study was the Habitable Zone Planet Finder, a spectrometer developed by a team at Penn State and affixed to the Hobby-Eberly Telescope at McDonald Observatory in Texas.

Using these instruments and others, the astronomers were able to uncover what they learned about K2-25b by observing the planet when it crossed in front of its sun, which it did every 3.5 days. Steffanson is hoping to use even more instruments to gather additional data.

“We are excited to continue to study the system,” he told us. “In particular, it would be very interesting to obtain future observations with the James Webb Space Telescope to constrain the atmospheric composition of the planet. Given the high density we infer, our prediction would be that K2-25b likely has a high mean-molecular weight atmosphere—which means it’s likely composed of elements heavier than hydrogen or helium—and could potentially have water and/or other high mean-molecular weight gases in its atmosphere.”

 The research has been published in the Astronomical Journal