University Astronomers Discover New Planetary System

Astronomers have identified an unusually dense rocky planet orbiting the red dwarf star LHS 1903, a finding that complicates prevailing theories of how planetary systems form and evolve. The discovery was made by a global research team co-led by scientists at the University of St Andrews and the University of Warwick.

LHS 1903 — also known as TOI-1730 — lies 116.3 light-years from the Sun in the constellation Lynx. The star has a mass of 0.538 solar masses and emits just 4.7% of its luminosity. It belongs in the “thick disc” population of the galaxy, meaning it is older than the Sun and was formed at a time when heavy elements were less abundant. The star hosts four closely orbiting planets, underscoring that substantial planet formation was already underway during those early, metal-poor conditions.

In many planetary systems around red dwarfs, dense rocky planets lie closer to the star, while planets farther out tend to have lower densities, suggesting the presence of thick gaseous envelopes. As Professor Andrew Cameron from the School of Physics and Astronomy at the University of St Andrews explained to The Saint, “Many compact systems of this kind feature dense, rocky planets close to the star, and planets with comparable masses but larger radii further out, indicating substantial gaseous atmospheres.”

The first three planets in the LHS 1903 system follow that pattern, “but the fourth is anomalous — nearly six times the mass of Earth, and far enough out that it should be able to retain a deep atmosphere, but with the density of an iron-silicate planet,” Cameron said.

Planet LHS 1903 e has a mass of 5.79 Earth masses and a radius of 1.73 Earth radii, producing a density of 6.1 grams per cubic centimetre, consistent with a rocky, iron-silicate composition. By contrast, planets c and d have significantly lower densities, 2.91 g/cm³ and 2.09 g/cm³, respectively, indicating likely gaseous envelopes. The innermost planet, LHS 1903 b, is also rocky, but with a density of 6.82 g/cm3.

Conventional models suggest that planets form as gaseous protoplanetary discs and gradually migrate inward. As they move through the disc, they can accrete thick atmospheres from surrounding gas. Cameron stated that “A gaseous disc orbiting a star has an outward pressure gradient that opposes gravity, making the gas orbit more slowly than a planet, which doesn’t feel the pressure. Planets forming within the disc thus experience a headwind, causing their orbits to decay slowly. Successive planets migrate in until stopped by gravitational interaction with their inner neighbours. In doing so, they accrete gas from the surrounding disc, forming deep atmospheres. Extreme UV irradiation from the star can strip the atmospheres off the inner planets, but it’s harder to do that for planet LHS1903 e, which is relatively far out.”

Cameron outlined two possible explanations: The planet may have migrated inward late, arriving “just as the disc ran out of gas, too late to accrete a deep atmosphere.” Alternatively, it may have lost a once-substantial atmosphere through a major collision. “The latter sounds fanciful,” he noted, “until you remember that the Earth-Moon system appears to be a product of just such a collision.”

With existing data, it is not yet possible to distinguish between these scenarios. Cameron emphasised that “it’s hard to differentiate between such hypotheses with the data we have,” but added that, viewed within the broader diversity of known planetary systems, LHS 1903 presents “a very interesting challenge to theory.”

Photo by Alisa Senses