What can rocky planets orbiting in the outer parts of a solar system teach scientists about planetary formation and evolution? This is what a recent study published in Science hopes to address as a team of scientists have discovered a rocky planet orbiting in the outer reaches of an exoplanetary system. This study has the potential to challenge longstanding hypotheses regarding the solar system architecture, specifically regarding rocky planets orbiting closer to their star and larger gas giants orbiting farther away.
For the study, the researchers analyzed four exoplanets in the LHS 1903 system orbiting a red dwarf star, the latter of which is smaller and cooler than our Sun. Due to the planets orbiting closer to their star than our planets orbiting our Sun, the researchers estimated the orbital periods for the four exoplanets were between 2.2 and 29.3 days. However, the researchers were surprised to discover that while the innermost planet was rocky and the second the third planets were gaseous, the outermost planet was also rocky. As a result, this finding contradicts longstanding notions about solar system architecture, specifically regarding our own solar system that rocky planets orbit closer to the star while outer planets are gaseous.
Artist's illustration of the LHS 1903 system. (Credit: ESA)
“We’ve seen this pattern: rocky inside, gaseous outside, across hundreds of planetary systems,” said Dr. Ryan Cloutier, who is an assistant professor in the Department of Physics and Astronomy at McMaster University and a co-author on the study. “But now, the discovery of a rocky planet in the outer part of a system forces us to rethink the timing and conditions under which rocky planets can form.”
The reason why rocky planets have been observed to form in the inner solar system while gaseous planets form in the outer solar system, as seen in our own solar system, is due to the distribution of gas, dust, and ice, with ice primarily existing in the outer solar system due to the much colder environment. However, the outermost planet in LHS 1903 is hypothesized to have formed in an ice-free environment, owing to its rocky nature.
How many more rocky worlds will scientists discover orbiting far out in solar systems in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!
Sources: Science, EurekAlert!