Tag Archives: radial velocities

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

EPIC-1166 b: a Neptune-mass planet with Earth-like density

How do planets form? Can they migrate through their solar system? What are they made of? What can modify a planet over time? Is Earth, or our solar system, special?

These are all questions that those in our field seek to answer. And there seems, to me at least, to be an easy way of figuring them out: Find More Planets.

NASAK2As last week’s news of 1200 new planets showed, the Kepler spacecraft is an excellent way of doing that. Even in it’s new and slightly more limited mode of “K2”, nearly 200 planet candidates and at least 50 bona fide planets have so far been detected.

I am involved in a collaboration between 7 European universities to search for and confirm planets in K2. So far this has resulted in half a dozen papers & planets including the 2-planet K2-19 system. Today I can add one more to that tally: EPIC212521166 b (or 1166 for short).

Finding and Confirming EPIC-1166 b

Initially, we searched the 28,000 stars observed by K2 in field 6; scouring the lightcurves with computer programmes and by eye to spot the repeated dips that might be the tiny signals of planets passing in front of their stars. A handful of candidates including 1166* stood out as promising targets, and we took those few stars to the next stage: radial velocities.

Transit Lightcurve EPIC1166
Transit Lightcurve of EPIC1166

Using the high-resolution spectrograph HARPS, we searched for the star’s to-and-fro motion that orbiting planets should create. In the case of 1166, we saw a strong signal on the same timescale as we expected from the transits.

Radial Velocities of EPIC-1166
Radial Velocities of EPIC-1166

Then, using a code called “PASTIS”, we modelled the radial velocities, the transit lightcurve and information about the star it orbits simultaneously to pin down exactly what 1166 could be. Almost unquestionably, it was a planet, which was a relief. But we can also tell the size of this planet: it has a radius of only 2.6±0.1 times that of Earth, but a mass a whopping 18±3 times our planet. Combined they give EPIC-1166 b a mass similar to Neptune but a radius more than 30% smaller.

Super-Earth or mini-Neptune?

This makes 1166 b a member of an interesting group of planets: between the size of our solar system’s largest terrestrial planet (Earth) and it’s smallest gas giant (Neptune). So which one of these does our planet most closely resemble?

Mass-Radius diagram showing EPIC-1166 compared to other exoplanets
Mass-Radius diagram showing EPIC-1166 compared to other exoplanets

From it’s density (5.7g/cm3), EPIC-1166b might seem to be closer to Earth than the puffy Neptune (1.64g/cm3). However, densities are misleading for objects so large. The high pressures in the interior of an 18 earth-mass (Me) planet are enough to crush rock and iron to much higher densities than their terrestrial values. This effect is so large that, for a 2.6Re planet to have earth-like composition (70% rock, 30% iron), it would need to be around 50 earth masses! That’s a density nearly three times higher than Earth’s, and clear evidence that 1166 b is not quite as Earthlike as first impressions.


Instead, it seems like our planet must contain something other than just rock and iron. The most obvious candidate is hydrogen gas. This is so light and fluffy that at atmosphere consisting of only 1% the mass of 1166 b (0.2Me) is enough to cover an 18Me earth-like core in a 0.4Re-deep atmosphere, and produce the mass and radius that we see. Alternatively, water could be another component that could drag the density down. For example, if 1166 b was 50% water and 50% rock, it could also explain the composition perfectly. However, this scenario is unlikely, and a hydrogen-dominated atmosphere seems to be the more likely option.

Getting a handle on the interior composition of a planet is interesting, but in EPIC-1166 b’s case it is especially perplexing. Planet formation models show that, once a planet grows to around 10Me, it should begin to rapidly draw in gas from the surrounding gas disc until it becomes a gas giant like Jupiter. In the case of 1166 b, we also have reason to think it likely migrated inwards to its current position through that very gas disc. This is because it is not close enough for tides to affect its position, and orbits in a circular (rather than eccentric) orbit; both pointers to disc migration.

So how did it avoid becoming a gas Giant? One way might be if EPIC-1166 b was a gas giant, but lost all its atmosphere due to UV and X-Rays emitted from its star. However, at 0.1AU and with a surface temperature of 600K (much less than many exoplanets), 1166 b is too far away to have been affected by activity.

impactMy favourite way of solving this puzzle (and it is pure speculation) is through giant impacts between planets. This could both grow a large planet at 0.1AU after the initial planet formation stage, and also blast away a large hydrogen atmosphere. The fact that the star is much older than the Sun (8±3 Gyr) and that we do not see any other planets in the system, further adds to the possibility that this was once a multiplanet system (like K2-19b and c), which destabilised, crashed together, and resulted in a single dense mini-Neptune.

The jury is still out on it’s precise formation. But with EPIC-1166 b orbiting a bright star, there is hope that we can re-observe the planet and tie down it’s size, composition and history even further. And, together with the diverse and growing crop of exoplanets, this new mini-Neptune will surely help to answer those important open questions in our field.

And if that fails we can always fall back on the exoplanet mantra: Find More Planets.

The paper was submitted to A&A and released onto arXiv (http://arxiv.org/abs/1605.04291) on May 13th 2016.

*EPIC-1166 b was initially (and independently) detected by Suzanne Aigraine and released on twitter.