With the phenomenal success of the Kepler mission and a proliferation of ground-based telescopes, we now know that planets are common in our galaxy. But the methods we’ve used to detect most of them are aimed at finding large planets orbiting close to their host stars. The farther a planet is, the less gravity pulls on the star and the less likely it is to align so that its orbit goes between that star and Earth, blocking out some starlight. Meanwhile, the focus has shifted to nearby stars as astronomers have begun building a catalog of targets for next-generation telescopes.
These numbers provide an intriguing backdrop to today’s announcement that one of the stars closest to Earth has a superterrestrial companion. Barnard’s star is a red dwarf only six light-years from our solar system; only the three stars of the Centauri system are closer. But the new planet orbits far enough from Barnard’s star that it had missed previous attempts. The detailed follow-up it discovered also hints at the possibility of a separate, more distant planet, and both could help inform our models of planet formation.
A new look
Barnard’s star has been widely observed over the years, partly because it is so close and partly because it is a prototype of a red dwarf star. These sightings included searches for exoplanets, but nothing about the system stood out. But unless you regularly observe a star, chances are you don’t happen to be looking at critical points in the planet’s orbit.
That seemed to be the case when a team of researchers began checking archival data for Barnard’s constellations. Their analysis suggested there may be a signal of something orbiting Earth with a 230-day period, but the data suffered from what the researchers call “very poor sampling.” This gave them time on a few telescopes to conduct an extensive monitoring campaign in 2016 and 2017, including simultaneous observations with more than one instrument.
The team was looking for signs that Barnard’s star was shifting back and forth due to a planet’s gravitational pull, an approach to planet-hunting called the radial velocity method. This would cause small changes in the color of the light from the Earth’s perspective due to the Doppler effect. If a planet brought about these changes, they would appear regularly in each of the planet’s orbits.
Analysis of Barnard’s star suggested it rotated about every 140 days (this could also cause changes as more or less active parts of the star rotate in and out of view). But a variety of methods of analysis suggested there was an additional signal after 233 days, with a false alarm probability of only one in ten.15. (There was one approach that suggested the signal was not real, but that analysis is known to produce false negative results.) The strength of the signal also increased as new observations were added, adding to the evidence that it is real. , reinforced.
A new planet – or two?
The greatest changes in a star’s radial speed will occur when the plane of the planet’s orbit is aligned with Earth. If this were the case with the newly spotted planet, it would have a mass 3.2 times that of Earth, putting it in the super-Earth category. If the orbit has a different orientation, the planet will be heavier. The timing of the signal indicates that the planet orbits about the same distance Mercury orbits our sun.
That location is interesting from a planetary science perspective. While Mercury is in a warm location in our solar system, Barnard’s star is much fainter, so conditions at that distance are much cooler there. So cool, in fact, that it’s right where some volatile chemicals, like water, would begin to freeze and form ice lumps — scientists call that distance the snowline.
While there are different snow boundaries for each chemical (carbon dioxide doesn’t freeze where water will), the presence of solids should dramatically change the dynamics of the disks that give rise to planets. Modeling has suggested that the snowline location may be the most efficient location to form a planet, which could then migrate elsewhere in the exosolar system in response to its interactions with the disk.
Could that have happened to Barnard’s star? It is standard practice during this analysis to remove all clues from a newly found planet and then reanalyze the data to see if additional signals are present. When the researchers did that here, a second, much weaker signal appeared at 6,600 days. Though they weren’t yet strong enough to rule out a false detection, the researchers figured out what it would mean anyway: a planet the size of Neptune orbiting a little closer than Jupiter’s orbits in our solar system. Even fainter signals were still present after that was removed, suggesting that Barnard’s star is worth tracking long term.
Even if the other potential planets turned out to be busted, the one that appears to be there would be close enough to us, and far enough from Barnard’s star, that we could directly image it using hardware within the following decade should come online. So somehow Barnard’s star is likely to make a lot of headlines in the coming years.
Nature2017. DOI: 10.1038/s41586-018-0677-y (About DOIs).
List image by IEEC/Science-Wave/Guillem Ramisa