University of Warwick astronomers, using an ESA telescope, have discovered a planetary system with a distant rocky world that turns our understanding of planet formation upside down
In our Solar System, the inner planets (Mercury to Mars) are rocky, and the outer planets (Jupiter to Neptune) are gaseous. This planetary pattern - rock then gas - is consistently observed across the Milky Way.
That was, until an international team of scientists, led by Dr. Thomas Wilson from the University of Warwick, took a closer look at a star called LHS 1903. Their observations, published in Science, reveal a system of four planets that breaks this convention.
The four planets of LHS 1903
The planets around LHS 1903 (a cool faint red dwarf star) begin with a rocky planet orbiting close by and then two gas worlds, the expected planetary pattern. However, using ESA's CHaracterising ExOPlanet Satellite (CHEOPS), the astronomers saw a surprising fourth planet at the system's outer edge - and the outermost planet was rocky, not gaseous.
"This strange disorder makes it a unique inside-out system" says first author Dr. Thomas Wilson, Assistant Professor in the Department of Physics, University of Warwick. "Rocky planets don't usually form far away from their home star, on the outside of the gaseous worlds."
Traditional models suggest that the closest planets to stars are rocky because stellar radiation sweeps away their gaseous atmospheres, leaving dense, solid cores behind. Gas giants form farther out in cooler regions where gas can accumulate, and planets can hold onto it. Yet the distant rocky world orbiting LHS 1903 appeared to have either lost its gaseous atmosphere or never formed one.
A late bloomer from another era
Dr. Wilson and his colleagues set out to explore various explanations for how this rogue rocky planet was formed. Could the rocky and gaseous planets have swapped places? Or had the rocky planet lost its atmosphere in a collision? They ruled out those theories.
Instead, they found evidence that the four planets did not form at the same time, as would be expected, but formed one after another in a process called inside-out planet formation.
If LHS 1903 gave birth to its four planets one after another from the inner to outermost planet, instead of bearing quadruplets at once, each planet evolves in turn, sweeping up nearby dust and gas and leaving further out worlds to wait their turn.
Dr. Thomas Wilson explains what this means for the rocky planet: "By the time this final outer planet formed, the system may have already run out of gas, which is considered vital for planet formation. Yet here is a small, rocky world, defying expectations. It seems that we have found first evidence for a planet that formed in a gas-depleted environment."
This small rocky outer planet might still be an odd exception, or it might be the first clue to a new pattern in how planetary systems evolve. Either way, it is a discovery begging for an explanation that lies beyond our typical understanding of how planets form.
Isabel Rebollido Research Fellow at ESA points out: "Historically, our planet formation theories are based on what we see and know about our Solar System. As we are seeing more and more different exoplanet systems, we are starting to revisit these theories."
Maximilian Günther, Cheops project scientist at ESA adds: "Much about how planets form and evolve is still a mystery. Finding clues like this one for solving this puzzle is precisely what CHEOPS set out to do."
