Barnard's Star is a small, dim star, of the type that astronomers call red dwarfs . Consequently, even though it is one of the closest stars to Earth, such that its light takes only six years to get here, it is too faint to be seen with the naked eye. Now, four small planets have been found orbiting the star. Teams in America and Europe achieved this challenging detection by exploiting precision instruments on the world's largest telescopes.
Author
- Coel Hellier
Professor of Astrophysics, Keele University
Diminutive Barnard's Star is closer in size to Jupiter than to the Sun. Only the three stars that make up the Alpha Centauri system lie closer to us.
The planets newly discovered around Barnard's Star are much too faint to be seen directly, so how were they found? The answer lies in the effect of their gravity on the star. The mutual gravitational attraction keeps the planets in their orbits, but also tugs on the star, moving it in a rhythmic dance that can be detected by sensitive spectrograph instruments. Spectrographs split up the star's light into its component wavelengths. They can be used to measure the star's motion.
A significant challenge for detection, however, is the star's own behaviour. Stars are fluid, with the nuclear furnace at their core driving churning motions that generate a magnetic field (just as the churning of Earth's molten core produces Earth's magnetic field). The surfaces of red dwarf stars are rife with magnetic storms. This activity can mimic the signature of a planet when there isn't one there.
The task of finding planets by this method starts with building highly sensitive spectrograph instruments. They are mounted on telescopes large enough to capture sufficient light from the star. The light is then sent to the spectrograph which records the data. The astronomers then observe a star over months or years. After carefully calibrating the resulting data, and accounting for stellar magnetic activity, one can then scrutinise the data for the tiny signals that reveal orbiting planets.
In 2024, a team led by Jonay González Hernández from the Canary Islands Astrophysics Institute reported on four years of monitoring of Barnard's Star with the Espresso spectrograph on the European Southern Observatory's Very Large Telescope in Chile. They found one definite planet and reported tentative signals that indicated three more planets.
Now, a team led by Ritvik Basant from the University of Chicago in a paper just published in Astrophysical Journal Letters, have added in three years of monitoring with the Maroon-X instrument on the Gemini North telescope. Analysing their data confirmed the existence of three of the four planets, while combining both the datasets showed that all four planets are real.
Often in science, when detections push the limits of current capabilities, one needs to ponder the reliability of the findings. Are there spurious instrumental effects that the teams haven't accounted for? Hence it is reassuring when independent teams, using different telescopes, instruments and computer codes, arrive at the same conclusions.
The planets form a tightly packed, close-in system, having short orbital periods of between two and seven Earth days (for comparison, our Sun's closest planet, Mercury, orbits in 88 days). It is likely they all have masses less than Earth's. They're probably rocky planets, with bare-rock surfaces blasted by their star's radiation. They'll be too hot to hold liquid water, and any atmosphere is likely to have been stripped away.
The teams looked for longer-period planets, further out in the star's habitable zone, but didn't find any. We don't know much else about the new planets, such as their estimated sizes. The best way of figuring that out would be to watch for transits, when planets pass in front of their star, and then measure how much starlight they block. But the Barnard's Star planets are not orientated in such a way that we see them "edge on" from our perspective. This means that the planets don't transit, making them harder to study.
Nevertheless, the Barnard's Star planets tell us about planetary formation. They'll have formed in a protoplanetary disk of material that swirled around the star when it was young. Particles of dust will have stuck together, and gradually built up into rocks that aggregated into planets. Red dwarfs are the most common type of star, and most of them seem to have planets. Whenever we have sufficient observations of such stars we find planets, so there are likely to be far more planets in our galaxy than there are stars.
Most of the planets that have been discovered are close to their star, well inside the habitable zone (where liquid water could survive on the planet's surface), but that's largely because their proximity makes them much easier to find. Being closer in means that their gravitational tug is bigger, and it means that they have shorter orbital periods (so we don't have to monitor the star for as long). It also increases their likelihood of transiting, and thus of being found in transit surveys.
The European Space Agency's Plato mission , to be launched in 2026, is designed to find planets further from their stars. This should produce many more planets in their habitable zones, and should begin to tell us whether our own solar system, which has no close-in planets, is unusual.
Coel Hellier has received research council grants for the discovery of exoplanets.
