New planet sheds light on young star system histories

Emmanuel Briseño

Two amateur astronomers helped researchers understand the evolution and migration patterns of planets.

Shortly after scientists publicly released data from the Kepler space telescope, citizen scientists Daryll LaCourse and Thomas Jacobs observed signs of a possible planet. They then contacted Andrew Mann, a research affiliate for the department of astronomy, who confirmed the existence with equipment from UT’s McDonald Observatory.

They found a new planet, called K2-25b, which orbits the red dwarf star K2-25 in the Hyades star cluster. The Hyades cluster is the closest open star cluster to Earth and represents the face of the Taurus constellation. An open star cluster is a group of a few thousand stars that were formed by the same molecular cloud.

“This [planet] is distinguished from many thousands of other planets we have found because of the fact that it is in this cluster,” Mann said. “For most stars around which we find planets, we don’t know how old they are, and most of them are probably old like the sun.”

The sun is about 5 billion years old, he said, while the star K2-25 is only about 600 to 800 million years old. This may sound old to college students, but it is actually young for a star. Mann said that the juvenile star and planet could help the researchers learn about how planets evolve over time. 

The planet K2-25b is about the size of Neptune, which is about four times the size of Earth. This is large compared to other planets that orbit red dwarf stars. The planet’s large size suggests that it may have an atmosphere composed of hydrogen and helium.

“Our hypothesis is that planets around younger stars start out as bigger, Neptune-sized objects, and they lose atmosphere over time,” Mann said.

Radiation from the star could eventually strip away the planet’s atmosphere, Mann said. For example, Mars may have had a similar, more gaseous atmosphere when it was a young planet, but now, its atmosphere is less than 1 percent as dense as Earth’s.

“Now we are actually going to see young planets that have bigger atmospheres, and we can extrapolate how quickly they lose them based on looking at planets of different ages,” Mann said. 

The researchers are also looking at the planet’s position relative to the star. If younger planets are orbiting farther away from their host star than their older counterparts, this could suggest planets migrate inward over their lifetimes.

Mann said that this could clarify astronomer’s ideas of how our solar system evolved and migrated. 

“The idea is you look at a bunch of planets that are young, in the millions or hundreds of millions of years,” Mann said. “And you compare them to one planet around a sun that is billions of years old and you learn how planets change.”