Tag Archives: earth

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.

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: