Astronomers have observed a planetary system that challenges current theories about planet formation, as it contains a rocky planet that formed outside the orbits of its gaseous neighbors, perhaps after most of the materials needed to form planets had been exhausted.
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 faint star known as a red dwarf, about 117 light-years from Earth in the direction of the constellation Lynx.
A light-year is the distance that light travels in one year, which is 9.5 trillion kilometers.
The star, called LHS 1903 , is about 50 percent the mass of the sun and five percent of its brightness.
What caught the attention of scientists was the arrangement of the planets. The nearest planet is rocky, followed by two gaseous planets, while the fourth planet, which current planet-formation theory suggests should be gaseous, is also rocky.
Astronomer Thomas Wilson of the University of Warwick in England, lead author of the study published in the journal Science, said: "The theory of planet formation states that planets close to their host star should be small and rocky, with little or no gas or ice."
He added: "That's because this environment is too hot to retain large amounts of gas or ice, and any atmosphere that forms there is likely to be blown away by the radiation from its host star. Conversely, it is believed that planets far from their stars form in cooler regions containing large amounts of gas and ice, creating gas-rich worlds with large atmospheres. This system contradicts this theory, as it presents us with a rocky planet outside the realm of gas-rich planets."
Wilson described this system as "an inside-out system".
In our solar system, the four inner planets are rocky while the four outer ones are gaseous. Rocky dwarf planets like Pluto, which orbit outside the gaseous planets, are much smaller than any major planet.
Since the 1990s, astronomers have discovered about 6,100 planets outside our solar system.
Researchers believe that the planets in this system did not form all at once in a large disk of gas and dust orbiting their host star, but rather formed sequentially, with the planets consuming the gas that was supposed to form the atmosphere of the fourth planet before it coalesced.
Wilson said the fourth planet was most likely one of the planets that formed in later stages.
For his part, astronomer and study co-author Andrew Cameron of the University of St Andrews in Scotland said: "Did it arrive by chance at the time when the gas ran out? Or did it collide with another object that stripped away its atmosphere? The latter possibility seems fanciful until we remember that the Earth-Moon system appears to be the product of such a collision."
Wilson said: "A temperature of 60 degrees Celsius is very similar to the highest temperature ever recorded on Earth , which is 57 degrees Celsius, so it is certainly possible that this planet is habitable. Future observations from the James Webb Space Telescope may reveal the conditions of this planet and help us understand how habitable it is," referring to the James Webb Space Telescope.
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