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Monday, December 5, 2011 / Duncan Forgan

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.

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