Will Dunham
WASHINGTON (Reuters) – Astronomers have observed a planetary system that challenges current theories of planet formation, in which a rocky planet formed outside the orbit of its gaseous neighbor, possibly after most of the planet-forming material was 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 constellation Lynx. A light year is the distance that light travels in one year, or 5.9 trillion miles (9.5 trillion kilometers).
The star, called LHS 1903, is about 50% as massive as the Sun and about 5% as bright.
The order of the planets caught the attention of scientists. The innermost planet is rocky, the next two planets are gaseous, and the fourth planet is what current planet formation theories suggest should be gaseous, not rocky.
“Planet formation paradigms suggest that planets close to their host star should form small, rocky planets with little gas or ice,” said Thomas Wilson, an astronomer at the University of Warwick in the UK and lead author of the study published in the journal Science.
“This is because this environment is too hot to sustain large amounts of gas or ice, and any atmosphere that formed would have been lost through radiation from the host star. Instead, distant planets are thought to have been built in colder regions with large amounts of gas and ice, which would have created gas-rich worlds with large atmospheres. This system challenges that by giving us a rocky planet outside of the gas-rich planets,” Wilson said.
Wilson calls it “a system built from the inside out.”
In our solar system, the four inner planets are rocky and the four outer planets are gaseous. Rocky dwarf planets like Pluto that orbit gas planets are much smaller than any planets in the solar system.
Since the 1990s, astronomers have discovered about 6,100 planets outside the solar system, called exoplanets.
All four planets in the newly observed system orbit closer to the sun than Mercury, our solar system’s innermost planet. In fact, the outermost planets orbit only about 40% of the distance between Mercury and the Sun. This is typical for planets orbiting red dwarf stars, which are far less powerful than the Sun.
The two rocky planets are classified as super-Earths, meaning rocky planets like Earth but with two to ten times the mass. The two gaseous planets are classified as mini-Neptunes, or gaseous planets that are smaller than Neptune, the smallest gaseous planet in the solar system, but larger than Earth.
Researchers suspect that the system’s planets did not form all at once in a giant disk of gas and dust orbiting its host star, but rather formed sequentially, or the gases that make up the fourth planet’s atmosphere were depleted by its sibling before merging.
The fourth planet is likely a “late bloomer,” Wilson said.
“It formed later than other planets in a gas-poor environment. There wasn’t really that much material to build the planet from,” Wilson said.
Another possibility is that it was born with a vast gaseous atmosphere that later disappeared in a catastrophe, leaving only a rocky planetary core.
“Did[the fourth planet]arrive coincidentally when it ran out of gas? Or did it collide with another body, stripping away its atmosphere? The latter sounds strange until you remember that the Earth-moon system appears to be the product of such a collision,” said study co-author Andrew Cameron, an astronomer at the University of St. Andrews in Scotland.
This fourth planet is also interesting because of its potential habitability. It has a mass 5.8 times that of Earth and a temperature of about 140 degrees Fahrenheit (60 degrees Celsius).
“A temperature of 60 degrees Celsius is very similar to the hottest temperature ever recorded on Earth, 57 degrees Celsius (135 degrees Fahrenheit), so the planet is definitely potentially habitable. Future James Webb Space Telescope observations could reveal the planet’s conditions and help us understand its habitability,” Wilson said.
(Reporting by Will Dunham; Editing by Rosalba O’Brien)
