Barnard’s Star Hosts a Frozen Super-Earth

Artist’s impression of the surface of a super-Earth orbiting Barnard’s Star
Image credit: ESO/M. Kornmesser

The image above shows an artists’ impression of what the surface of the super-Earth orbiting Barnard’s Star might look like. However, given that the planet receives only about 2% of the light Earth receives from the Sun, this image is probably over-illuminated.

After many decades of debate, controversy, and sometimes-acrimonious refutations of data, it turns out that the closest single star to us, Barnard’s Star, does appear to host what seems to be a rocky planet, after all. Designated V2500 Ophiuchi, Barnard’s Star is a cool M-class dwarf star that is at least 10 billion years old. This implies that the 3.2 Earth-mass planet is almost certainly much older than Earth.

Nonetheless, while the planet orbits Barnard’s star at a distance of only 0.4 AU, with an orbital period of 233 days, the planet is located outside of the star’s habitable zone. This is apparent given the host star’s surface temperature of only just more than 3000K. To put this in perspective, the planet’s surface temperature is estimated be as low as -1700C (-2750F). Needless to say, this makes the possibility that the planet is hosting life, or has hosted life during its long history, vanishingly small.

Moreover, the possibility that the (hypothetical) planet is habitable is further reduced by the fact that all M-class stars are flare stars. In the case of Barnard’s Star, the last observed flare occurred on the 17th of July 1998. Subsequent analysis showed that the flare reached a maximum temperature of about 8,000K- more than twice the average surface temperature of the star.

These types of flares are roughly analogous to the eruptions that occur on the Sun. However, the process as it relates to M-class stars is not well understood. However, Barnard’s Star is generally relatively stable, which is thought to be the result of it having lost most of its original rotational energy during the past 10-12 billion years or so. Therefore, the mechanisms that produce flares either have stabilized, or are not as active as before.

As a rule, the flares that most M-class stars produce are extremely energetic. Consequently, it is entirely possible that if the planet orbiting Barnard’s Star did have an atmosphere, or frozen water or other ices on its surface, these would have been boiled off into space by repeated flares long ago. This is especially true given its very close proximity to the star.

Discovered Using Global Array Of Telescopes

Regardless of the planets’ physical characteristics though, finding the planet, and confirming its existence was not easy,. This is despite the star being located only about six light years away. In a paper published in the journal Nature, the lead author, Ignasi Ribas (Institute of Space Studies of Catalonia and the Institute of Space Sciences, CSIC in Spain), states that even after 20 years of measurements and observations using an array of instruments that spanned the globe, there is still only a 99% possibility that the planet actually exists.

In fact, the large possibility that a planet is orbiting Barnard’s Star is largely due only to the extreme accuracy of the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph. The instrument is able to measure deviations in a star’s radial motion that are as small as 3.5km/hour, which is a leisurely walking pace. By combining HARPS measurements with more than 700 measurements from at least a dozen other equally sensitive instruments located around the world, the research team was only able to conclude that it is likely that the observed variations in the brightness of Barnard’s Star was caused by an orbiting body, rather than by inherent changes in the star’s luminosity.

Nonetheless, the authors of the paper point out that their findings do not amount to incontrovertible proof that a planet is indeed orbiting Barnard’s star. In fact, the authors are at some pains to qualify their findings by stating “…we’ll continue to observe this fast-moving star to exclude possible, but improbable, natural variations of the stellar brightness which could masquerade as a planet.”

In the final analysis though, it hardly matters whether or not a planet is orbiting Barnard’s Star. For one thing, even six light years is beyond our reach for the purposes of exploring the planet. What this exercise does prove, though, is that it has become possible to detect ever-smaller exoplanets. This is a most noteworthy achievement in the ongoing hunt for exoplanets in general, and Earth-analogues in particular.

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