PlanetWatch: GJ1214b confirmed by Hubble as a Waterworld

It’s difficult to blink without seeing a press release on an unusual exoplanet.  This time it’s GJ1214b who’s back in the headlines.  It was discovered by the Mearth project in 2009, using the transit method.  Mearth purposefully looks at M type stars, i.e. very low mass stars, because smaller planets will be able to block out a larger fraction of the host starlight.

The Mearth Project, composed of 8 16" telescopes controlled robotically. Credit: Cfa

When GJ1214b was discovered, the team were able to constrain its mass between one and ten Earth masses from the period of the planet’s orbit. Combined with the direct measurement of the planet’s radius (2.7 Earth radii), the Mearth team were able to infer that the planet’s density was close to that of water.

This is more than just a big quivering blob of liquid.  The core of this planet would presumably have some very exotic forms of water that exist only at very high pressures, such as Ice VII, which has a bizarre crystalline structure compared to regular ice, or superfluid water, which acts as if it has no friction or viscosity.

Even regular, old-fashioned steam would seem to exist here.  In 2009, theoretical models underestimated the apparent radius of GJ1214b, which the Mearth team interpreted as the presence of a steamy atmosphere above the surface.

Now, 3 years later, the Hubble Space Telescope has been brought to bear on GJ1214b.  Hubble’s Wide Field Camera 3 was trained on the planet as it went through a transit.  As the planet obscures its host star, starlight shines through the planet’s atmosphere towards Earth.  The atmosphere preferentially absorbs at specific wavelengths, leaving a chemical fingerprint in the light’s spectrum.

An artist's impression of GJ1214b transiting its host star. Credit: CfA

The fingerprints might still be a little smudgy, but data suggests the atmosphere is at least 20% water, probably much higher.  The best guesses for the composition of the planet would appear to be an icy, rocky core with a very watery atmosphere and a healthy pinch of hydrogen and helium.  How exactly the planet ended up quite so watery is unclear – it would almost certainly have had to form quite far from its host star, beyond the distance at which ices melt (usually called the snow line), and migrated inwards.

Could it be inhabited? A large surplus of water in a variety of phases is certainly a boon for life on Earth, and we know how diverse and teeming the oceans are.  But bear in mind the temperature on the surface was estimated to be around 200 degrees C.  This would certainly lead us to say that terrestrial life would struggle to live here, but the oceans of GJ1214b will be weird to say the least, containing bizarre ice and superfluid water…if there was life, it would have to be pretty weird too.


PlanetWatch: The Hunt for Exomoons

Over the Christmas break, I saw this paper pop up in my Twitter feed, which caught my eye. The Hunt for Exomoons with Kepler (HEK) is an ambitious attempt to do something that has never been done before – detect a moon orbiting an exoplanet.

Now, let’s put the audacity of this venture in perspective. Exomoons are pretty much invisible in radial velocity (or Doppler Wobble) measurements, unless you have a superhuman spectrograph that’s absurdly sensitive to minute shifts in velocity. The best way to detect them is via transits (hence the current attempt to use Kepler). You’re unlikely to “see” the moon itself. What you’re more likely to see is the host exoplanet’s transits “wobbling”, as the moon tugs on the planet (see pic below).

Diagram of the star-planet-moon system (moon not shown, taken from Kipping 2009)

The first type of “wobble” is referred to as Transit Timing Variation (TTV), where the period of the transit (i.e. the time interval between two transits) changes with a well-defined sine curve. The second is Transit Duration Variation (TDV), where the length of time a single transit takes changes.

TTV is pretty good, but it is degenerate: this means that it can only measure the exomoon mass and orbital radius as a combined entity, not separately. TDV is also degenerate in a similar way, but measuring both signals together allows scientists to disentangle mass and orbital radius. Transit variations can be used to detect non-transiting planets in the same system as transiting planets, but this has never been achieved for exomoons.

HEK will perform an automated analysis of all planet candidates found by Kepler (over 2300 at last count), and search for positive detections. Even the null detections will allow an estimation of what fraction of exoplanets have a massive moon, which will be an important value for planet formation theorists to understand and explain.

The HEK project is likely to be our best shot of achieving exomoon detection. We expect Kepler to be able to detect exomoons down to about 0.2 Earth Masses. These are pretty massive moons compared to the Solar System (for comparison, Jupiter’s most massive moon, Ganymede, is 0.025 Earth masses), but detecting such a tiny object at such large distances would be a triumph of modern astronomy.

Lastly, I should note that the HEK paper is headed up by none other than David Kipping, who is a friend and colleague (we began our PhDs in the same year). I look forward to seeing his name pasted across the papers when the first exomoon is detected!

PlanetWatch: Kepler-22b, and Why We Need HARPS North

The week of the Kepler Science Conference (or #KepSciCon, to the Twitterati) was always going to be momentous, with another release of data from the space telescope’s science team.  And they didn’t disappoint! Kepler-22b is the closest we’ve come to seeing a twin Earth in the Milky Way.

Image Credit: NASA/Ames/JPL-Caltech

Details on the Kepler-22 system are frustrating thin – it’s too soon for the data to appear in the web archives such as exoplanet encyclopaedia, so we have to rely on the scraps of data that appear in the press release.

Kepler detects exoplanets via transits (where the planet eclipses its parent star, and the starlight dips in intensity).  Because of this, Kepler measures the area of the planet, relative to the star’s area.  As the star is a typical G type (like our Sun), the team can calculate the absolute size of the planet, which has a radius about 2.4 times that of Earth (the above graphic shows the planets approximately to scale – you can see how Kepler-22b dominates over the others).  They can also measure the period of the orbit, which is about 290 days (cf Venus, whose orbit is 224 days).  This might seem like bad news for Kepler-22b-ites, but thankfully the star, Kepler-22, is about 25% less luminous than the Sun.  If we placed Earth where Kepler-22b is, the temperature would (apparently) be a rather nice 22 degrees Celsius.

But remember that Kepler-22b has a radius 2.4 times the Earth’s.  If it has a similar chemical composition, the mass of Kepler-22b could be about 14 times the mass of Earth.  Surprisingly, this still makes it quite a low-mass planet – a Super-Earth – and this value is probably an upper limit.  Without any knowledge of the planet’s composition, scientists will have to guess its mass.

The only way we’ll be able to confirm the mass of this planet is to carry out a follow-up survey, using radial velocity (or Doppler Wobble) techniques. These directly probe the gravitational forces in the system, and by extension the masses.  And it just so happens that Edinburgh University is part of the HARPS North consortium, which plans to build an instrument to do just that.

The Telescopio Nazionale Galileo in the Canary Islands, destination of the HARPS-North instrument (Source: University of St. Andrews)

The Kepler Space Telescope is ploughing an exciting furrow in exoplanet research, and telescopes like the one above will be following in its path, honing and refining its discoveries.  HARPS-North will certainly be in demand: after all, I doubt this is the last habitable planet that Kepler will see.  They have 54 candidate habitable planets still in the pipeline, awaiting confirmation, and that list can only grow.  We’ve never been closer to seeing our sister world in the skies, and we’re about to get closer still.