Rocky Outer Planet Upends Planet Formation Ideas

Astronomers have found four planets around the red dwarf star LHS 1903, 116 light-years away in the constellation Lynx. The system starts with a rocky planet close to the star, followed by two gas-filled worlds, but the farthest planet turns out to be rocky too. This unusual setup, spotted using telescopes on Earth and in space, goes against the common pattern seen in most planetary systems, including our own Solar System. The discovery came to light through detailed observations published this week.

Background

Most planetary systems follow a simple layout. Rocky planets like Earth sit near their star, while bigger gas giants like Jupiter circle farther out. This comes from how stars and planets form. A disk of gas and dust surrounds a young star. Close to the star, heat and radiation from the star's light strip away any gas around forming planets, leaving just rock and metal behind. Farther out, where it is cooler, planets can hold onto thick layers of gas and grow into giants.

Our Solar System matches this. Mercury, Venus, Earth, and Mars are rocky and close in. Jupiter, Saturn, Uranus, and Neptune are gas giants farther away. Hundreds of other systems observed across the galaxy show the same thing. Astronomers have built models around this idea. They say the disk around a star cools over time, but the inner parts stay hot enough to prevent gas buildup.

LHS 1903 is different. It is a small, cool red dwarf star, much dimmer than the Sun. Red dwarfs make up most stars in the galaxy, so studying their planets matters a lot. Teams have watched this star for years. At first, they found three planets that fit the standard pattern: one rocky inner world and two mini gas giants after it. But new data changed the picture.

Key Details

The star LHS 1903 sits in the Milky Way's thick disk, a older part of the galaxy. It is about half the size of the Sun and much fainter. The planets orbit close to it, with periods from 2.2 days for the innermost to 29.3 days for the outermost.

The Planets in Order

• LHS 1903b: The closest planet, rocky, with a dense core of rock and iron.
• LHS 1903c: A gas world, like a mini Neptune, with a thick atmosphere.
• LHS 1903d: Another gas planet, also holding onto its gaseous envelope.
• LHS 1903e: The surprise, farthest out and rocky, with no sign of gas around it.

Scientists used the European Space Agency's CHEOPS satellite, along with ground telescopes, to measure the sizes and masses of these worlds. Transit photometry tracked how the star's light dimmed as planets passed in front. Radial velocity measurements caught the star's slight wobble from the planets' pull. These methods showed LHS 1903e sits in what is called the radius valley, a gap between rocky super-Earths and gassy sub-Neptunes.

The team ruled out other ideas. A big crash could not have stripped away gas from LHS 1903e, as models did not match. Planets switching places over time also failed the tests. Instead, the evidence points to inside-out formation. The planets built up one after another, starting from the inside.

"We've seen this pattern: rocky inside, gaseous outside, across hundreds of planetary systems. 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." – Ryan Cloutier, McMaster University

Each planet cleared out dust and gas nearby as it grew. By the time LHS 1903e formed last, the area around it had less gas left. Cooler conditions far out usually favor gas giants, but without enough gas, it stayed rocky.

What This Means

This system shows planets can form in stages, not all at once. The disk around LHS 1903 changed over time. Early planets took most of the gas, leaving scraps for later ones. This inside-out process explains the odd order: rocky, gas, gas, rocky.

It challenges the one-size-fits-all model of planet formation. Red dwarfs like LHS 1903 have long lives and stable disks, so their systems might evolve slowly. This could be common for small stars, even if rare overall. Future observations might find more such setups.

The radius valley in exoplanet sizes now has a live example within one system. LHS 1903e formed without a gas envelope from the start, in a gas-poor zone. This supports ideas that some planets never get atmospheres, while others lose them.

"The most compelling theory to explain the rocky planet says that LHS 1903 gave birth to its four planets one after another from the inner to outermost planet, instead of bearing quadruplets at once." – Annelies Mortier, University of Birmingham

Researchers now plan more looks at LHS 1903 and similar stars. Instruments like the James Webb Space Telescope could check atmospheres directly. Better data on masses and orbits will test the formation timeline. If more outer rocky planets turn up, it could reshape how we think planets build across the galaxy.

This find highlights gaps in our knowledge. Planet formation stays hard to watch up close, as it happens fast around young stars. Systems like LHS 1903, now mature, offer snapshots of old processes. The work involved teams from McMaster University, University of Warwick, University of Birmingham, and ESA. Their combined data from CHEOPS and other tools made the detection possible.