A small red dwarf star in the Milky Way has drawn attention after astronomers mapped four closely orbiting planets around it. The system, known as LHS 1903, does not follow the layout many scientists associate with planet formation. Their worlds range from rocky to gas-rich, and they sit across a size divide known as the radius valley. The arrangement appears reversed when compared with our own solar system. Researchers say the outermost planet is rocky despite forming later than its inner neighbours. The finding adds detail to an ongoing debate about how small exoplanets form and evolve, particularly around older red dwarf stars in the thick disc of the Milky Way.
Scientists spot a solar system they say should be impossible
Astronomers studying distant worlds have noticed a pattern. Most small exoplanets fall into two categories. Some are super-Earths, rocky planets slightly larger than Earth. Others are sub-Neptunes, larger bodies that tend to hold thick gaseous atmospheres.Between these groups lies a gap in size called the radius valley. Planets rarely fall within this range. Researchers think intense heating from a host star can strip away a planet’s atmosphere, pushing it into the rocky category. Another view is that some planets never gathered much gas at all. The LHS 1903 system offers a compact example of this divide. The four planets orbit very close to their star, completing full orbits in periods ranging from just over two days to about 29 days.Density measurements show variation. The innermost planet appears dense and rocky. The next two have lower densities, suggesting extended atmospheres. The outermost planet, however, shows no sign of a thick gaseous envelope. That detail stands out.
Observations combine space and ground telescopes
According to a report by CNN, the system was first identified using NASA’s Transiting Exoplanet Survey Satellite. The satellite detects slight dips in starlight when planets pass in front of their star.Follow-up observations came from the European Space Agency’s CHEOPS, designed to study known exoplanet systems in greater detail. Ground-based telescopes also contributed data. The project involved researchers from several countries.Transit photometry helped figure out how big planets are. Small changes in the star’s speed were used to measure its radial velocity and find its mass. Scientists were able to figure out density and makeup by using all of these methods together.
An inside-out formation challenges expectations
In our solar system, rocky planets such as Mercury, Venus, Earth and Mars orbit close to the Sun. Gas giants like Jupiter and Saturn sit much farther out.The LHS 1903 system does not mirror that order. Researchers tested whether collisions could have stripped gas from the outer planet. Simulations did not support that idea. Instead, they suggest a gas-depleted formation process. In this model, planets formed one after another, starting near the star and moving outward.By the time the outermost planet formed, much of the surrounding gas and dust had dispersed. With little material left, it remained rocky. The result does not overturn existing models. It complicates them slightly. Planet formation appears less uniform than once thought.