Tag Archives: harps

Proxima Centauri b

After a week of controversy and embargo-breaking, the actual science behind the detection of Proxima Centauri b is finally here (published in Nature). And it, honestly, is a breathtaking discovery. A terrestrial planet around the closest star to our sun. It proves what Kepler showed: Earth-like planets really are everywhere, including around the star next door. But should we believe it? And is it all that it is hyped up to be?

The star:

As you can probably tell from the name, Proxima is the closest star to Earth. Located only 4 lightyears away in the Alpha Centauri system, it is a tiny red speck of light, only visible in a telescope. The reason for it’s lackluster brightness is that the star itself is dimunative. Only 12% of the size of the Sun, it is also 100 million times fainter. Although that may sound bizarre, M-dwarfs like it are the most common type of star in our galaxy.

The signal:

HARPS at the 3.6mMany people have hunted for planets around Proxima before. These usually involve monitoring the star’s radial velocity, it’s to-and-fro speed, and searching for the tell-tale tug of a gravitationally bound exoplanet. But until 2016, there had been no luck. That’s when the Pale Red Dot team decided to throw everything they could at the star to try to do what others had not.

Using the HARPS instrument on La Silla (which I am currently sat only 50m from), they took observations nearly every night for 3 months. And, as we found out yesterday, that kitchen-sink technique paid off. They found a 1.5m/s (that’s brisk walking pace) with an 11.2 day signal. And it had a 99.9999% chance of being real. And they found the same signal, hidden just below detectability in the past data too.

A strong signal in the HARPS data


Activity and detection:

When the rumours were flying, I urged caution on this potential discovery. One of the reasons being that Proxima is not a quiet sun-like star. It is instead a turbulent M-dwarf. That manifests itself in large star-spots, strong stellar flares and varying shapes in the spectral lines (the bar-code like lines we observe in the colours of the star). All of these cause confusion in the radial velocities, and there have been a few planets (some of which were discovered by this very team) which are now assumed to be simply variability.

But, they have convinced me. One way they have done this is with simultaneous photometry. That means not just observing the star with a spectrograph, but also simultaneously measuring its brightness with an imaging telescope. This photometry also gives a view of the activity of the star, but without any of the doppler signal from the planet. And what the team see is that the photometry (the trends in brightness) matches up perfectly with the activity that is suggested by certain features in the spectra. And that this signal is completely different to that from the planet.

So, I have to say it: it seems unlikely that the strong signal comes from the star itself, and much more likely that we are indeed seeing the gravitational tug of an orbiting planet.


Firstly, we only have a minimum mass for the planet. What this means is that, it could not be less than 1.3Me, but it certainly could be more. That is because the signal from a small planet with its orbit observed edge-on has the same signal as a larger planet observed more obliquely (pole-on). So do not be surprised if it turns out to be larger than this first measurement

M-dwarfs and habitability:

kepler_438bAnother caveat is that the planet probably isn’t habitable. I know that flies in the face of every news headline, but hear me out. Firstly, as I’ve said before, Proxima Centauri throws out an abundance of flares. These are so numerous and so strong that they are clearly seen four times in the ~80 nights of photometry. With a planet only 0.05AU away (1/20th the distance of Earth), these flares would have the potential to do damage to any organic molecules on the surface. The paper itself suggests the sterilising X-ray flux could be 400 times that experienced by Earth; and are likely to have been much higher in the past.

Another problem is that any body that close to another, larger body is likely to be tidally locked. Just look at the moon. This proves problematic for habitability. The large temperature gradient from day to night a tidally locked planet sucks the atmosphere (with supersonic winds) to the cold side of the planet. There, atmosphere can gets frozen and be lost. You can break this cycle, but that involves having a very thick (and equally un-earthlike) atmosphere.

Further planets:

One interesting remark was that there seems to be another signal in the data from a more massive outer planet. Now, this signal might be closer to the rotational (and therefore activity) cycle of the star so could more easily be a false positive. But it would not be surprising if, like our own terrestrial planet, it had bigger siblings lurking slightly further out.


As with any exoplanet result, it seems like everything besides a few key details is speculation ( I have even seen some press speculating on the number of continents proxima has!). In fact, details such as its true mass aren’t completely tied down just yet. And even 1.3Me planets can still be un-earthlike; look at KOI-314c for example.

But, unlike most of the ‘earthlike’ planets we have found, there’s a pretty good chance we could actually answer these questions directly. And I don’t just mean with giant telescopes (although those would obviously work too) – I mean actual in situ observations. Crossing 4 lightyears of space currently no more than a pipe-dream, but it’s not inconceivable to think that, within our lifetimes, a probe might set off to see just how earthlike these exoplanets really are. And there’s no question where it will be going first; towards a Pale Red Dot…

For more information visit the Pale Red Dot website: http://www.palereddot.org
For more information visit the Pale Red Dot website: http://www.palereddot.org

The Solar System’s has Four New Neighbors

The number of worlds discovered around other stars is now counted in the thousands. But, if you were to go out on a dark night and try to spot those planet-hosting stars with your own eyes, you would struggle – only 6% of planets orbit stars bright enough for our eyes to pick out. This is especially true of transiting planets; those that pass in front of their star relative to our line of sight. Of more than 1000 such planets known, only one (55 Cancri) is bright enough to see in the night sky. That is, until today…

Position of HD219134 in Stellarium

HD 219134, nestled between Cassiopeia and Cephus, is remarkable in so many ways. It was first studied with HARPS-N, during it’s Rocky Planet Search. This instrument, a spectrograph on the TNG telescope in the Canary Islands, is able to measure the motion of stars so precisely that it can spot the to-and-fro wobble caused by planets.

Amazingly, this instrument found not just one but four planets around this star; a mini solar system just like our own. The outermost is a gas giant on a 3-year orbit, while the inner three are between the size of Earth and Neptune orbiting once every 3, 7 and 47 days.

And the prize for funkiest Colour scheme goes to...
And the prize for funkiest Colour scheme goes to…

ssc2015-02b_Inline[1]At this point, astronomers had no idea if these new worlds transited. But a planet on a 3-day orbit has pretty good odds to pass in front of its star so, taking control of the Spitzer space telescope, they pointed it and hoped. And sure enough, exactly when predicted, the innermost planet blocked out 0.036% of starlight. This fraction is just the surface area of the star covered up, giving a precise measure of the radius of the planet.

Now, with the mass of the planet measured by HARPS and the radius of the planet measured by Spitzer, it’s density can be found. While many similar sized worlds have turned out to be fluffy gas-balls rather than true super-Earths, a density of 5.89gcm-3 puts HD 219134b bang on Earth-like composition. If there was a surface, it’s gravity would be just under twice what we experience on Earth (18.8ms-3). With an orbit of only three days, though, the planet’s star-facing surface is likely to be hot enough to melt!


At only 20 light years away, the newly-discovered solar system around HD219134 is also the closest transiting exoplanet ever found, and one of the 20 closest bright star systems to our Sun. With transiting planets extremely rare, there’s even a chance that this could actually be the closest transiting planet around a bright star (K & G-type).

HD219134’s brightness is also important for astronomers. The brighter & closer a planet, the more interesting ways we can study it. For example, this new world has jumped to the top of the list for those trying to study exoplanet atmospheres. We can also measure the path it takes as it crosses it’s star to determine just how the planet orbits. The outer 3 planets might peturb the orbits of the inner one, causing detectable variations in transit timing (TTVs).

It has truly been a remarkable week for exoplanet astronomy, beginning with the discovery of habitable-zone super-Earth Kepler-452b, and now the detection of the brightest, closest, awesomest transiting planet ever found. And, thanks to a huge array of exciting follow-up options, this will not be the last you’ve heard of HD219134b,



Here’s how you can find the star in the sky (and a very neat animation of the transit):


The paper by Montelabi can be found on arXiv here

Other coverage includes:

Elizabeth Tasker’s piece on “The closest rocky planet ever has been found… so what?

Sci-News: HD 219134: Three Super-Earths Found Orbiting Star 21 Light-Years Away

Daily Mail: Star discovered with THREE Super-Earths, and one is the closest rocky planet ever found outside our solar system Read more: 

Gliese 581d is an ex-planet

Exoplanet poster child

If, in 2009, you asked 18-year-old me to name an exoplanet, then Gliese 581d would have been it. Discovered by an American team of astronomers in 2007, it was, for a long time, the poster child for exoplanetary science. Not only was the first rocky world ever found in the habitable zone of its star where life-friendly temperatures are found, it was also relatively nearby (for astronomy standards) at only 20 light years.

Astronomers used the radial velocity technique to find the first planet around Gliese 581 as far back as 2005. This method relies on the gravitational pull that a planet has on a star as it orbits. This wobble is detectable in the spectra of the starlight, which gets doppler shifted as the star moves back-and-forth, allowing the period and mass of an orbiting planet to be determined. While the first planet, ‘b’, orbited close to the star with a period of only 5.4 days, it was joined by two cooler (and more habitable) planets, ‘c’ and ‘d’ in 2007. This was soon followed in 2009 by Gliese 581e, the smallest planet in the system on an even shorter (3.1d) orbit.

Movie credit: ESO

Things started to get even more confusing in 2010 when observers at the Keck observatory announced two more planets (‘f’ and ‘g’) orbiting at 433 and 37 days respectively. This would put ‘g’ between ‘c’ and ‘d’ and right in the middle of the star’s habitable zone. However, new observations of the star with a Swiss telescope showed no such signal. Was there a problem with the data, or could something else be mimicking these planets?

Other stars, just like our sun, have extremely active surfaces
One problem comes when we consider the star itself. Just like our own sun, most stars are active, with starspots skimming across the surface and convection currents in the photosphere causing noise in our measurements. These active regions can often mimic a planet, suppressing the light from one side of the rotating star and shifting the spectra as if the star itself were moving back-and-forth. Add to that the fact that, like planets, activity comes and goes on regular timescales and that cool stars such as Gliese 581 are even more dynamic than our pot-marked sun, and the problem becomes apparent.

The first planet to bite the interstellar dust was ‘f’. At 433 days, its orbit closely matches an alias of the star’s 4.5-year activity cycle, and it was quickly retracted in 2010. Similar analyses with more data also suggested Gliese 581g was also likely to be an imposter, but the original team stuck by this discovery. For the last 3 years, this controversy has simmered, until last month all the data available for Glises-581 was re-analysed by Paul Robertson at Penn State. This showed that not only is Gliese 581g not a planet, but that the poster child itself, Gliese 581d, was also an imposter.

The signal strength of any potential planets with (red) and without (blue) activity correction.

To do this, the team took all 239 spectra of GJ581 and analysed not just the apparent shift in velocity, but the atomic absorption lines themselves. Using the strength of the Hα absorption line as an indicator for the star’s activity, they compared this to the residual radial velocity (after removing the signal from planet b). This showed that there was a relatively strong correlation between activity and RV, especially over three observing seasons when the star was in a more active phase. They also found that this activity indicator varied on a 130 day timescale.

The new system with only 3 planets
When the team removed the signal from stellar activity, they found that planets ‘c’ and ‘e’ were even more obvious than in previous searches. However the signal for planet ‘d’ dropped by more than 60%, way below the threshold needed to confirm a planet. Even more remarkably, ‘g’ does not appear at all. So what exactly caused this ghostly signal. The planet’s orbital period of 66 days gives us a clue -it is almost exactly half that of the star’s 130 day rotation cycle, so with a few fleeting starspots and the right orientation, a strong planet-like signal at 66 days results.

This case of mistaken identity is a sad one, but thanks to the incredible progress of our field in the last 5 years, their loss barely makes a dent in the number of potentially habitable exoplanets known. Instead, it acts as a warning for planet-hunters: sometimes not all that glitters is gold.

The results are also explained in exquisite detail at Penn State University’s own blog, including an excellent timelapse showing how our understanding of the Gl 581 system has changed over time