Astronomers puzzle over 'inside out' planetary system

Astronomers have observed a planetary system that challenges current planet formation theories, with a rocky planet that formed beyond the orbits of its gaseous neighbors, possibly after much of the planet-forming material had been used up.

The system, observed using the European Space Agency's Cheops space telescope, consists of four planets - two rocky and two gaseous - orbiting a relatively small and dim star called a red dwarf about 117 light-years from Earth in the direction of the Lynx constellation. A light-year is the distance light travels in a year, 9.5 trillion km.

The star, named LHS 1903, is about 50% as massive and 5% as luminous as our sun.

The order of the planets is what caught the attention of scientists. The innermost planet is rocky, the next two are gaseous and the fourth one, which current planetary formation theory suggests should be gaseous, instead is rocky.

"The planet-formation paradigm states that planets close to their host star should form small and rocky, with little-to-no gas or ice," said astronomer Thomas Wilson of the University of Warwick in England, lead author of the study published in the journal Science.

"This is because this environment is too hot to maintain substantial gas or ice, and any atmospheres that do form are likely removed via irradiation from their host star. Conversely, planets at larger separations are thought to be built in colder regions with a lot of gas and ice that would create gas-rich worlds with large atmospheres. This system challenges that by giving us a rocky planet outside of gas-rich planets," Wilson said.

Wilson called it "a system built inside-out."

In our solar system, the four inner planets are rocky and the four outer planets are gaseous. The rocky dwarf planets like Pluto that orbit beyond the gas planets are much smaller than any of the solar system's planets.

Astronomers have detected about 6,100 planets beyond our solar system, called exoplanets, since the 1990s.

All four in the newly observed system orbit closer to the star than our solar system's innermost planet Mercury orbits the sun. In fact, the outermost planet orbits at only about 40% of the orbital distance between Mercury and the sun. This is typical for planets orbiting red dwarf stars that are so much less powerful than the sun.

The two rocky planets are categorised as super-Earths, meaning rocky like Earth but two to 10 times more massive. The two gas planets are categorised as mini-Neptunes, meaning gaseous and smaller than our solar system's smallest gas planet Neptune but larger than Earth.

The researchers suspect that rather than forming all at once in a large disk of gas and dust swirling around their host star, this system's planets formed sequentially, with gas that otherwise would have made up the atmosphere of the fourth planet being used up by its sibling planets before it coalesced.

Wilson said the fourth planet most likely was a "late bloomer."

"It formed later than the other planets in a gas-poor environment. There was actually not so much material to build this planet," Wilson said.

Another possibility is that it was born with a large gas atmosphere that later was lost in a calamity, leaving behind just the rocky planetary core.

"Did (the fourth planet) arrive coincidentally just as the gas ran out? Or did it suffer a collision with another body which stripped its atmosphere away? The latter sounds fanciful until you remember that the Earth-moon system appears to be a product of just such a collision," astronomer and study co-author Andrew Cameron of the University of St Andrews in Scotland said.

This fourth planet also is interesting because of its potential habitability. Its mass is 5.8 times that of Earth and it is about 60 degrees Celsius.

"A temperature of 60 degrees Celsius is very similar to the hottest temperature recorded on Earth, 57 degrees Celsius, so it's definitely possible that this planet is habitable. Future James Webb Space Telescope observations could reveal the conditions of this planet and help us understand how habitable it might be," Wilson said.