Astronomers puzzle over ‘inside out’ planetary system

Astronomers discover an unusual planetary system with a rocky planet formed outside its gaseous neighbors, challenging existing formation theories.

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.