The planets orbiting one of the stars closest to us have turned out to be unsuitable for life. They orbit far too close to their host star and almost certainly lost their gaseous envelopes billions of years ago. In addition, their interiors are composed largely of a mineral that does not retain water well.
The Chemistry of Their Interiors
The chemical composition of a star can provide clues about what its planets are made of. A team from the University of Cambridge analyzed the star’s spectrum and found an abnormally high abundance of magnesium compared with other stars of the same type.
As a result, the interiors of all four planets are expected to be dominated by periclase, a rare mineral based on magnesium oxide. On Earth, it occurs only at depths of several hundred kilometers.
Lead author Xander Byrne of Cambridge’s Institute of Astronomy explains that on our planet, magnesium is used mainly to form olivines, which are capable of retaining moisture within the interior. Periclase cannot do this. The results were published in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society.
Orbits That Are Too Close
All four worlds orbit at distances equal to only one to four percent of Earth’s orbital radius. Even the farthest of them is located ten times closer to the red dwarf than Mercury is to the Sun.
Under such conditions, the planets are almost certainly tidally locked and permanently face the star with the same side. The dayside of each planet has been exposed to radiation and stellar flares for ten billion years, which is the age determined through spectral analysis.
Where Their Atmospheres Went
Their gaseous envelopes lasted for approximately two billion years, after which radiation pressure stripped them away into space, according to Universe Today. The weak gravity of these planets, which are less massive than Earth, was unable to retain the gas under the constant stream of particles emitted by the star.
Xander Byrne puts it simply: the planets were always likely to be hostile to life because their low gravity, combined with their close orbits, left their atmospheres with no chance of survival.
Stable Orbital Period Ratios
Compact planetary systems are often unstable because of the gravitational interactions between their planets. The planets may either collide with one another or be ejected from the system and become free-floating worlds.
Such an outcome is unlikely here because the three inner planets move in a stable orbital-period ratio of 9:12:16. A similar mechanism operates among Jupiter’s Galilean moons Io, Europa, and Ganymede, whose orbital periods follow a 1:2:4 ratio.
What Scientists Will Search for Next
Until now, bodies of this size have largely escaped astronomers’ attention because larger worlds are much easier to detect. Xander Byrne expects the sensitivity of new instruments to reduce this observational bias, allowing rocky planets the size of Earth or smaller to be found increasingly often.
The next step will be the launch of PLATO, the European Space Agency’s telescope designed to search for planetary transits and stellar oscillations. The spacecraft will conduct long-term observations of bright stars and detect even the smallest variations in their brightness.
The paradox is that the first claimed discovery around this same star was announced as early as the 1960s and involved a supposed gas giant. Peter van de Kamp’s astrometric measurements were later explained as the result of an equipment defect, while the planets that actually exist turned out to be thousands of times smaller.
