With all of the new discoveries that seem to be making almost monthly, it is sometimes hard to remember that finding exoplanets is still a relatively new and difficult science. As part of these ongoing discoveries, a team announced back in 2018 that it had found a planetary candidate around Barnard’s star, one of the closest to ours. Now another team has reanalyzed the data, collected some new data, and determined that the planet detection was likely false positive.
A variety of factors have been suggested as the cause of the false positive. However, the easiest thing to do is to consider the method of detection, the star’s period of rotation, and a signal processing artifact.
The team that originally discovered the planet was known as CARMENES, a consortium of Spanish and German astronomers. They performed their due diligence and monitored the star over a period of 23 years using 7 different data collection tools. In fact, they weren’t the first group to suggest that the star system, second closest to our own, might have a planet. In 1963, an astronomer named Peter van de Kamp claimed to have found a planet around the star in what was then the first discovery of an exoplanet. This finding was later unmasked by a large number of researchers who also used the astrometry method, but could not reproduce the “wobble” found by van de Kamp.
Discussion of the astrometry method of exoplanet hunting.
Photo credit: ESA YouTube Channel
Fast forward to 2018 and the CARMENES team announce their new planet around Barnard’s star, which they found using the radial velocity method (RV). This method relies on the slight blue and red shift that occurs when a star approaches or retreats from us, caused by the gravitational pull of a planet it orbits.
Some factors confuse these RV measurements. One that appears to have caused the false positive around Barnard’s star results from the star’s period of rotation. Barnard’s star has an extremely slow rotation – about 145 days, which is almost six times longer than the Sun’s 25-day period of rotation.
Barnard’s star is close enough that it can actually move against the stellar background over the decades. It has the fastest rate of motion of any star
Photo credit: Steve Quirk
This period of rotation is important for several reasons. First, sunspots are difficult to account for in RV measurements because they can easily be interpreted as a displacement. To take this into account, scientists usually collect data for more than one revolution in a row to see if they can track the sunspot as it rotates around the star. Unfortunately, the total observation time of Barnard’s Star is only about 270 days, which means that scientists cannot collect data over two full revolutions at the same time. Sunspots can also exist on M dwarf stars (of which Barnard is one) for more than 10 revolutions, so that sunspots can influence the RV values of the star even over several observation periods.
According to a team at the Habitable Planet Finder (HPF), that’s exactly what happened. What she first pointed out to this possibility was the orbital period of the proposed planet. After 233 days it could be detected during a single observation window. However, there is a quirk of signal processing called aliasing that makes this period unlikely.
Example of the aliasing of a periodic signal. If samples are only taken at points 1,5 and 9, one of the two signals will match the data.
Photo credit: Andrew Jarvis, Wikimedia Commons
Periods of 145 days (the star’s rotation period) and 233 days (the proposed planet’s orbit) are aliases of each other. Aliasing occurs when data is only sampled intermittently. This is exactly what happens when ground-based telescopes can no longer observe the star due to the earth’s rotation or its position around the sun – hence the 270-day observation window mentioned above.
When a signal is sampled “sparingly”, the resulting data can be fitted by several sinusoidal patterns. These sparsely sampled data points can also be affected by the sunspots noted above. In fact, they can do so in ways that can create a false positive that the HPF team believes happened in the case of the exoplanet at Barnard’s star.
UT-Video Video in which Barnard’s star is partly discussed as the closest star in the northern hemisphere.
To prove their point, they re-analyzed the old data and also collected new data about the star. The new data collection has been simplified, as astronomers typically use the relatively stable Barnard’s star as a calibration tool for commissioning their instruments. With this new dataset, they couldn’t find any evidence of a planet. When they re-analyzed the data and compared it to three different expected models of the system. The first would be if the system had a planet with no star activity (i.e. sunspots), the second would be if there was a planet but there would also be star activity, and the third would be if there was no planet but still star activity.
Using Occam’s razor, they found that the third model, lacking a planet, best fits the entire data set. All of this means that things don’t look good for a planet to be at Barnard’s star.
UT video about finding exoplanets and the new telescopes used to find them.
The CARMENES team themselves suggested that this might be possible and stated in their original paper and press release that they could not be 100 percent certain of the presence of a planet. As part of the scientific process, they can review the paper written by the HPF team and determine if they agree with their conclusions or provide some other explanation for their original finding.
Another aspect of this entire exercise is that scientists must exercise caution when calibrating new highly sensitive instruments. Even standard calibration tools such as Barnard’s Star, used for decades, are active at a level that the most delicate instruments made today can detect. If this activity is ignored, planets around a neighbor aren’t the only false positives emanating from these new instruments.
PSU – A very furtive pseudonym: the cheat planet of Barnard’s Star
arXiv – Stellar activity manifested in a year-long alias, explains Barnard b as false positive
Forbes astronomers rule out super-earth around Barnard’s star
S&T – A team of astronomers with new data and analysis deny claims that a super-earth orbits near Barnard’s star
UT – Amateur Astronomer Chases Barnard’s Star – You Can Too!
Artistic impression of the putative exoplanet, which may not exist.
Photo credit: ESO / M. Kornmesser