'Upside Down' Planetary System Challenges Astronomers

Astronomers have discovered a rare planetary system that challenges current theories of planet formation by identifying a rocky planet orbiting beyond its gas-rich neighbors.

The system was observed using the European Space Agency’s CHEOPS space telescope. It consists of four planets, two rocky and two gaseous, orbiting a relatively small and faint red dwarf star known as LHS 1903. The star is located about 117 light-years from Earth in the direction of the Lynx constellation.

LHS 1903 has roughly 50 percent of the Sun’s mass and about 5 percent of its brightness. A light year is the distance light travels in one year, approximately 9.5 trillion kilometers.

What surprised scientists most is the unusual order of the planets. The innermost planet is rocky, followed by two gas planets. However, the fourth and outermost planet, which current planet formation theory predicts should also be gaseous, is rocky instead.

Thomas Wilson, an astronomer at the University of Warwick in England and lead author of the study published in Science, said standard theory suggests planets close to their host star are typically small and rocky with little or no gas or ice.

He explained that the environment near a star is too hot to retain large amounts of gas or ice, and any atmosphere formed there is likely to be stripped away by radiation. In contrast, planets farther from their stars are expected to form in cooler regions rich in gas and ice, creating large gas dominated worlds with thick atmospheres. This newly discovered system does not follow that pattern.

Wilson described the system as being built from the inside out.

In our Solar System, the four inner planets are rocky while the four outer planets are gas giants. Although rocky dwarf planets such as Pluto orbit beyond the gas giants, they are much smaller than the major planets.

Since the 1990s, astronomers have identified about 6,100 exoplanets, or planets outside our Solar System. However, systems arranged in this way remain uncommon.

Researchers believe the planets in this system did not all form at the same time from a single disk of gas and dust. Instead, they may have formed sequentially. The gas planets could have consumed much of the material that would otherwise have formed a thick atmosphere around the outer rocky planet.

Wilson suggested that the fourth planet likely formed at a later stage. Co author Andrew Cameron of the University of St Andrews in Scotland said another possibility is that the planet may have experienced a collision that stripped away its atmosphere. He noted that the Earth Moon system is believed to have formed through a similar massive impact.

Scientists estimate the planet’s surface temperature at around 60 degrees Celsius, close to the highest temperature ever recorded on Earth at 57 degrees Celsius. While extreme, researchers say it does not rule out the possibility that the planet could be habitable.

Future observations using the James Webb Space Telescope are expected to provide more details about the planet’s atmosphere and environmental conditions, offering new insight into how such unusual planetary systems form and whether they could support life.