Interviewer: Benjamin Thompson

For our first story today listeners, we’re going to travel into space for several trillion kilometres, or about six light years. This is the distance to Barnard’s star, one of the closest stars to our Sun after the Alpha Centauri system. Barnard’s star is a red dwarf and is relatively cool and small. It’s about a sixth of the mass of our Sun. Due to its close proximity, astronomers have spent decades searching Barnard’s star for evidence of orbiting exoplanets. There were claims in the 60s that a planet had been discovered, but these were later disproven and other searches have so far drawn a blank. However, things might be about to change, as a group of researchers have combed through a huge amount of previous data and think they might have found evidence that an exoplanet is there after all. Now, you might think that searching for an exoplanet would be easy – just point a telescope at a star and have a look. According to Ignasi Ribas, who led the new research, it’s not that simple.

Interviewee: Ignasi Ribas

Yeah, wouldn’t it be nice to have a digital camera take a telescope and take a picture and then see a nice planetary system orbiting a star, right? This would be our dream, and this dream is what we call direct detection of exoplanets.

Interviewer: Benjamin Thompson

Ignasi, who is based at the Spanish Research Council and the Institute of Space Sciences of Catalonia, hasn’t fulfilled this particular dream quite yet. In fact, direct detection is often pretty difficult for a number of reasons, so astronomers use indirect detection methods instead, looking for the effects that an exoplanet is having on its star. One of these indirect detection methods is known as radial velocity, and this is what Ignasi and his colleagues have used in their latest work. Radial velocity looks at a star’s slight wobble as it is tugged by an orbiting exoplanet. Now, you can’t see the actual movement of this wobble from Earth but as the star moves towards or away from us, you can detect a tiny, repeating shift in the frequency of its light, which we can see as its colour. This frequency shift indicates the size and period of the wobble.

Interviewee: Ignasi Ribas

That’s an indirect indication that indeed there’s an orbiting object around that star and then from this wobble we can actually calculate how massive is the object that is responsible for this wobble, and this technique has been used for ages.

Interviewer: Benjamin Thompson

Ignasi combined hundreds of radial-velocity measurements of Barnard’s star, collected by different groups over the past 20 years. Within these combined data, he and his colleagues identified a low-amplitude signal that repeated every 233 days.

Interviewee: Ignasi Ribas

And this tells that there could be a potential planet there with really low mass because the wobble is really small, and with a period that’s quite long. It’s comparable to the year of our year actually. We’re talking about 233 days – that’s really long compared to most of the other planets that people have observed. And by collecting all this huge amount of data, we have managed to see this signal very strong in the data actually.

Interviewer: Benjamin Thompson

The team reckon that the exoplanet’s distance to Barnard’s star is a bit less than half the distance of Earth to the Sun. They also suggest its mass is a minimum of 3.2 Earth masses. This puts the exoplanet in the super-Earth category. Now, sadly this doesn’t mean that it’s home to superheroes, but it does mean that it’s between 1 and 10 Earth masses. Other than that, we don’t really know much more about it.

Interviewee: Ignasi Ribas

The sad thing about finding planets with radial velocity is that we know very little about the planets actually. To tell you the real honest answer, is that we know how long its year lasts and we know what the minimum mass is. Everything else we have to calculate or speculate.

Interviewer: Benjamin Thompson

So, too early to talk about the exoplanet’s terrain or the inevitable discussion about its chances of harbouring life. In fact, perhaps it’s too early to talk about a lot of things. Is there really a planet there at all? After all, when it comes to Barnard’s star and exoplanets, astronomers have been wrong before. Rodrigo Diaz from the Institute of Astronomy and Space Physics in Buenos Aires, Argentina, has written a News and Views article about this new work. He says that when looking for exoplanets using radial velocity, there are a few pitfalls to be aware of.

Interviewee: Rodrigo Díaz

You have to be sure that the activity of the star is not tricking you because stars are not just balls of light emitting light uniformly, but they have their own activity and they have their own variations, and some of this can actually mimic velocity variations. So, you can confuse activity of the star, usually related to the magnetic fields of the star and the rotational rate of the star, with an actual planetary signal.

Interviewer: Benjamin Thompson

Rodrigo said that the amount of data analysed in this new research was impressive, and there’s reason to be optimistic that there is an exoplanet orbiting Barnard’s star. However, he noted that there’s still a chance that this could be a false positive.

Interviewee: Rodrigo Díaz

As the authors know perfectly well, this is a very tough thing to detect, so to test the robustness of the detection they used different methods to account for the fact that the star is active and that the star has its own intrinsic variability. So, one of these methods came out with a kind of worrying result that the signal that they see could actually be produced by the star. But it’s one method in a few, but then again you know that when you’re doing this kind of reasoning, the reasoning is as strong as your weakest link, so if there’s a weak link in the chain then we have to take the result with caution.

Interviewer: Benjamin Thompson

Ignasi told me that he’s 99% confident that an exoplanet is present, but he says there’s still work to be done.

Interviewee: Rodrigo Díaz

We have to keep gathering more data to make the case even stronger, and this 99% go to 99.9%, to 99.999% if we can, right? So, we’re trying to nail this and to make sure we investigate the star deeply enough to make sure this activity that all stars have is not the culprit of this signal that we see.

Interviewer: Benjamin Thompson

Both Ignasi and Rodrigo hope that in future, other detection methods and things like the soon-to-be-launched Gaia space observatory could confirm the presence of the exoplanet and tell us about its composition and atmosphere. There is so much to learn about how super-Earth-sized planets form, so this one – a mere hop, step and a jump away from us in astronomical terms – could be a good place to start. You can read Ignasi’s paper and Rodrigo’s News and Views article over at nature.com/nature.ⓝ