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!
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.
It’s hard to know what to say about Occupy Wall Street (and its sister movements across the world) that hasn’t already been said. That’s partially why I haven’t written anything about it yet. As the movement settles into its third month of action, making its way to Washington D.C. through seemingly horrific scenes of apparent police brutality, it continues to undergo sustained attacks from the conservatives (with a lower case c).
I’ve been pretty ambivalent about the whole thing, to be honest. I think the reason I haven’t been able to come to a conclusion about them is because of how nebulous the whole thing is. Apart from the admittedly pithy “we are the 99%”, I haven’t been able to divine the purpose of #OWS.
