Astonished astronomers reported Thursday the discovery of a star whose planets are arranged in an unusual sequence that challenges established scientific understanding, indicating these distant worlds may have formed in a way never observed before.
In our Solar System, the four planets nearest the Sun are small and rocky, while the four outer planets are gas giants.
Until now, scientists believed this pattern—rocky planets first, followed by gas giants—was a universal rule.
However, a star named LHS 1903, located in the Milky Way’s thick disc, tells a different story.
An international team of astronomers, using data from multiple telescopes, had previously identified three planets orbiting this red dwarf, which is cooler and dimmer than our Sun.
In this system, the innermost planet is rocky, with two gas giants farther out—matching the arrangement scientists typically anticipate.
However, a closer analysis of data from Europe’s exoplanet-hunting Cheops space telescope uncovered a fourth, more distant planet in the system—and it is rocky.
“This creates an inside-out configuration, with the planets arranged as rocky, gaseous, gaseous, and then rocky again,” said Thomas Wilson, lead author of a new study detailing the discovery in the journal Science.
“Planets made of rock typically don’t form so far from their host star,” the planetary astrophysicist from the University of Warwick in the UK said in a statement.
Planet by Planet
Inner planets are usually small and rocky because strong radiation from the nearby star strips away most of the gas surrounding their solid cores.
Further from the star, in the colder regions of the system, planets can retain thick atmospheres around their cores, giving rise to gas giants.
Perplexed by the unusual arrangement of the LHS 1903 system, the team of astronomers set out to understand what might have caused it.
After eliminating several explanations, they proposed a new idea: what if the planets formed sequentially, one after another?
The prevailing theory holds that planets form all at once within a massive ring of gas and dust known as a protoplanetary disc. In this process, tiny dust grains stick together, gradually growing into cores that eventually develop into full-fledged planets.
However, by the time the fourth planet around LHS 1903 came into existence, “the system may have already depleted its gas,” Wilson explained.
“Yet here is a tiny, rocky planet that goes against all expectations,” he added.
“It appears we have discovered the first evidence of a planet forming in what we describe as a gas-depleted environment.”
Since the 1990s, astronomers have identified over 6,000 planets beyond our Solar System—known as exoplanets—primarily by detecting small dips in starlight as the planets pass in front of their stars.
“Traditionally, our theories of planet formation have been shaped by what we observe in our own Solar System,” said Isabel Rebollido, a planetary disc researcher at the European Space Agency.
“But with the growing number of diverse exoplanet systems we’re discovering, it’s becoming necessary to rethink these theories.”
Read the original article on: Sciencealert
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