Astronomers have observed a planetary system that challenges existing theories of planetary formation: it contains a rocky planet that formed beyond the orbits of its gaseous "neighbors," possibly after much of the material for planet formation had already been consumed.
The system was observed using the European Space Agency's (ESA) CHEOPS space telescope, and consists of four planets - two rocky and two gaseous - orbiting a relatively small and dim star, a red dwarf, about 117 light-years from Earth in the direction of the constellation Rhea, Reuters reports.
A light year is the distance that light travels in one year - 9,5 trillion kilometers.
The star, called LHS 1903, has about 50 percent of the Sun's mass and only about 5 percent of its luminosity.
It's the arrangement of the planets that has caught the attention of scientists. The closest to the star is a rocky planet, the next two are gaseous, and the fourth - which the theory of planet formation suggests should be gaseous - is rocky instead.
"The planet formation paradigm says that planets close to their host star should form small and rocky, with little or 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 the environment is too hot to hold significant amounts of gas or ice, and any atmospheres that do form are likely stripped away by radiation from their host star. In contrast, planets at greater distances are thought to form in cooler regions, with lots of gas and ice, creating 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 called 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, which orbit outside the gaseous planets, are significantly smaller than any planet in the solar system.
Astronomers have discovered about 6.100 planets outside our solar system, called exoplanets, since the 1990s, Reuters reports.
All four planets in this newly observed system orbit closer to their star than Mercury orbits the Sun. In fact, the outermost planet is only about 40 percent of the orbital distance between Mercury and the Sun. This is typical for planets around red dwarfs, which are much fainter than the Sun.
The two rocky planets are classified as "super-Earths" - rocky like Earth but two to ten times more massive. The two gaseous planets are "mini-Neptunes" - gaseous and smaller than Neptune, the smallest gaseous planet in our system, but larger than Earth.
Researchers suspect that, rather than forming simultaneously in a large disk of gas and dust swirling around the star, the planets in this system formed one after the other - with the gas that would otherwise make up the fourth planet's atmosphere being consumed by the "sister" planets before it even formed.
Wilson said the fourth planet was most likely a "late bloomer."
"It formed later than the other planets, in a gas-poor environment. There really wasn't much material to build this planet," Wilson said.
Another possibility is that it was born with a large gaseous atmosphere, which was later lost in some catastrophe, leaving behind only a rocky core.
"Did (the fourth planet) arrive by chance just as the gas was running out? Or did it experience a collision with another body, stripping its atmosphere? The latter sounds fanciful until you remember that the Earth-Moon system appears to be the product of just such a collision," said astronomer and study co-author Andrew Cameron of the University of St Andrews in Scotland.
The fourth planet is also interesting because of its potential habitability. Its mass is 5,8 times that of Earth, and its temperature is around 60 degrees Celsius.
"A temperature of 60 degrees Celsius is very similar to the highest temperature ever recorded on Earth - 57 degrees Celsius - so it is definitely possible that this planet is habitable. Future observations with the James Webb Space Telescope could reveal conditions on this planet and help us understand how habitable it might be," Wilson said.
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