Recreation of the landscape of the exoplanet Barnard b orbiting its star. Image © IAC/ Gabriel Pérez.
Astronomers Jonay González Hernández and Alejandro Suárez Mascareño of the Instituto de Astrofísica de Canarias (IAC), have discovered an exoplanet orbiting Barnard’s star, the closest single star to our Sun. It is an intriguing plot twist given just such a planet was the subject of a 1934 science fiction novel called The Legion of Space.
Using ESPRESSO (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations), a third generation, fibre-fed, cross-dispersed, echelle spectrograph mounted on the European Southern Observatory’s Very Large Telescope (ESO’s VLT), their observations also hint at the existence of three more exoplanet candidates, in various orbits around the star.
Located just six light-years away, Barnard’s star is the second-closest stellar system – after Alpha Centauri’s three-star group – and the closest individual star to us. Owing to its proximity, it is a primary target in the search for Earth-like exoplanets. But despite a promising detection back in 2018, no planet orbiting Barnard’s star had been confirmed until now. The discovery of this new exoplanet is the result of observations made over the last five years with ESO’s Very Large Telescope, located at Paranal Observatory in Chile. ScienceVega spoke to González Hernández and Suárez Mascareño to find out more about Barnard’s star and the potential planets in its orbit.
ScienceVega: Why did you choose to study Barnard’s star?
JGH: Barnard’s star is one of the closest stars to our Sun and is currently of interest to the exoplanet community in science. We have been using the ESPRESSO instrument, which has gone further than other instruments have gone before, and it means we have achieved more sensitivity to see Earth-like planets.
With the start of exoplanet science we tried to explore everything; all the stars we could see and all the stars we could reach. But the science is now focusing on the closest stars. The reason for that is that in the future, following on from the studies where we are able to find planets in the closest stars, then we may be able to do more studies in the near future with new facilities like the Extremely Large Telescope (ELT) now being built in Chile. We may also be able to use space missions that will be launched in the future, perhaps in the 2040s and 50s.
Going in this direction, ESPRESSO is the most precise instrument in the world right now to try to find these kinds of planets. Our observations began in 2018 when we started by looking at the closest stars, the first being Proxima Centauri. We were able to confirm an Earth-like planet there in the habitable zone, and we were able to see another smaller planet in the inner orbit. The next nearest star after Proxima Centauri is Barnard’s star.
ASM: One important thing is that with future facilities, we will try to characterise planets in the nearby star systems. The reason to choose nearby stars is because most of the exoplanets are not transiting, where they pass in front of their star relative to us, so we will need techniques that rely on the thermal emission of the planet or the reflected light. With the ELT we will be able to separate that from the starlight.
But to do that we need the exoplanet to be separated from the star. It depends on both the orbital separation and the distance to us, because it’s basically a perspective effect. Therefore in the future, under this separation, we will only be able to do it in nearby stars. That means nearby stars are really important and in this case, being the second closest system to us is one of the easiest ones in which we can do this.
One thing that is not usually taken into account when finding projects, is that Barnard’s star has a history that makes it a very compelling target to actually solve. It was one of the first targets where astronomers claimed there were planets in the 1960s. Those were eventually false positives, but scientists have been trying to solve the Barnard’s star planetary system for at least 60 years. It has been a difficult puzzle to solve, but we are now the closest we’ve ever been to doing it.
This diagram illustrates the locations of the star systems closest to the sun. The year when the distance to each system was determined is listed after the system’s name. NASA’s Wide-field Infrared Survey Explorer, or WISE, found two of the four closest systems: the binary brown dwarf WISE 1049-5319 and the brown dwarf WISE J085510.83-071442.5. NASA’s Spitzer Space Telescope helped pin down the location of the latter object. The closest system to the sun is a trio of stars that consists of Alpha Centauri, a close companion to it and Proxima Centauri. Image © NASA/ Penn State University, Public domain, via Wikimedia Commons.
ScienceVega: There was a false positive detection of an exoplanet orbiting Barnard’s star in 2018. Can you outline the process in terms of only now being able to confirm that?
JGH: The closest stars are the ones that we want to analyse but we have to deal with some peculiarities of the stars which are really difficult to tackle such as the magnetic activity of the star. Typically the magnetic activity of the star maybe due to spots on the surface and the rotation of the star, so we are seeing a surface with different temperatures and this asymmetric structure is rotating, which we have to take into account. Typically these effects are ten times larger than the exoplanets that we are measuring; the effect of this star activity is having consequences that are ten times stronger than the signals of exoplanets.
We are trying to look for low mass exoplanets and we are now sure that there are no big exoplanets orbiting Barnard’s star.
So in 2018, we were trying to find the planet that was published but now, and this sometimes happens in science when you make a discovery and then other studies try to confirm it, the data we have now with ESPRESSO is significantly better than the data we had before and this data is not supporting the existence of that candidate planet we published in 2018. It was down to the orbital period and orbital distance which was very different from the planet we are now detecting. So the first detection had an orbital period of 233 days, whereas this one is 3.15 days. It is a completely different planet; the first was bigger at perhaps three times the mass of Earth and this one is perhaps a few times the mass of Mars, or half the mass of Venus, which makes it significantly lower in mass than the Earth.
We are now convinced that the latest planet is confirmed and the previous one was no more than a candidate. It is the rotation which is causing the signal which cannot be confused with the signal of a planet.
These techniques are very complicated and everything needs to be taken into account. You need to organise the observations to really be able to see both effects and to be able to model all the effects. Before we were trying to push the instrument to its limits and because of the different tools that we were using there were several factors that were not really ideal for the research. In the end this caused the detection that we were thinking was there, but now we are sure that it is not.
ASM: The 2018 planet most likely does not exist. It was mostly an artefact of the stellar rotation. We are talking about one confirmed planet and potentially three companions, all of them in short orbits near the star.
This artist’s impression from 2018 shows the proposed super Earth orbiting Barnard’s star. González Hernández’s and Suárez Mascareño’s latest research used data from ESPRESSO that is significantly better than the data used previously, and is not supporting the existence of that candidate planet. Image © ESO/M. Kornmesser, CC BY 4.0 < https://creativecommons.org/licenses/by/4.0 >.
ScienceVega: What can you tell me about the other exoplanets you may have found?
JGH: The only thing that we can be sure of is that the planet is too close to the star to have liquid water on the surface because it is too hot. Even if the star is 2,000°C cooler than the Sun, the planet is so close that it is too hot to have liquid water on the surface, so it is not in the habitable zone. The only parameter that we have measured is what we call ‘minimum mass’. We measure the mass of the planet using a technique that is looking at how the star is oscillating back and forth in the line of sight.
This is the only thing that we can measure. By estimating the mass of the star and the measurement of the velocity signal of the star, we are able to measure the mass of the planet. We know that the minimum mass of this planet is a few times the mass of Mars, so it’s really a small planet.
With ESPRESSO we have now seen planets that are smaller than the Earth; we call them ‘sub-Earths’. We don’t know what they are. In this case we don’t know if this planet has an atmosphere or not. What we are sure of is that the planet is so close to the star that it is always facing the same way, just like our moon. And in this sense, we expect that the side that is facing the star is much hotter than the side facing the opposite.
From the TESS (Transiting Exoplanet Survey Satellite) data we have been analysing we have been trying to find a transit, but we know there is no transit so the inclination of the orbit is such that the planet is not passing in front of the star.
ASM: If we discuss the full system there is one interesting point in that the potential orbits, which we need to confirm, they would be in a resonance chain. There are a few systems like this orbiting M dwarf stars where the orbital period of the planets is such that each is in proportion to the previous. Theoretically this is complicated. There are systems that are like that and systems that are not. The Solar System is not like that, but the system of the moons of Jupiter is. With low mass stars there are quite a few that are like this. For example, the Trappist-1 system is like this but others are not, and theoreticians are working really hard to find out why.
We don’t think we have a clear answer yet.
This artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018. Image © NASA/ JPL-Caltech, Public domain, via Wikimedia Commons.
ScienceVega: What research would you like to be doing following on from this particular study?
JGH: First we want to confirm the other three candidate planets we have already proposed. Hopefully if our proposal, which we did in September, is approved then we will be able to study the star again next year (2025) when it is visible in the sky. By the end of next year, if all goes well, we may be able to confirm the other planets or at least some of them.
The other thing is that having a planet in the closest star systems is always something that allows us to think about how to explore these planets in more detail. For instance there is a project of building a very big telescope to try to get an image of these planets. We never see them unless they are transiting and what we see is not actually the planet itself, but how the light of the star is dimmed as the planet crosses.
So we would like to have images of these exoplanets, these small sub-Earths. We are trying to go in this direction to detect an Earth-like planet and perhaps in the future we may be able to image them.
ASM: We have to wait until we have confirmed whether there are four planets, or three or one, but one thing we have thought about doing is using the James Webb Space Telescope to see whether they have atmospheres or not, even if they don’t transit. If they don’t transit, then one side of the planet is like a mirror and it gets really hot and the other side gets really cold. But if the planet has an atmosphere, it distributes the heat and the difference is no longer that big.
When you have the planet orbiting around a star, sometimes you see the bright side, sometimes you see the dark side, and you can measure that light over the stellar light with James Webb or some other telescopes. That can tell you whether a planet has atmosphere or not. It’s not much more informative, but you can get to that point. It is something we will try in the future, probably, and if we don’t do it, someone else will!
Something else to add is that Barnard’s star has the fastest apparent motion of any star and we have known that for a long time. In the 20th Century there was a science fiction novel called The Legion of Space which talked about humans travelling to Barnard’s star and returning to tell of a gigantic planet with all kinds of aliens and monsters. I really liked it as a kid. It’s really cool to see that we are finding planetary systems in the same systems that science fiction dreamed about having planetary systems.
JGH: By analysing these stars and now realising that most of the stars have planets, with the increase in the sensitivity that we now have to detect those planets, it may be possible that there is a planet in the habitable zone that we have not yet been able to see.
This artist’s impression shows Barnard b, a sub-Earth-mass planet that was discovered orbiting Barnard’s star. Its signal was detected with the ESPRESSO instrument on ESO’s Very Large Telescope (VLT), and astronomers were able to confirm it with data from other instruments. An earlier promising detection in 2018 around the same star could not be confirmed by these data. On this newly discovered exoplanet, which has at least half the mass of Venus but is too hot to support liquid water, a year lasts just over three Earth days. Image © ESO/ M Kornmesser. CC BY.
