The first K2 observation of a transiting exoplanet

My life has been consumed recently with planning for the K2 mission (a mission where Kepler observes into the ecliptic). The Senior Review proposal to NASA has just been submitted and we have received over 100 proposals from community scientists to observe targets during our first science campaign.


K2 managed to catch one transit of WASP-28b during a short test observation in Jan 2014. The data were obtained at 1 min intervals.

We have some data on the ground and the good news is that we can detect planets! We have observed our first exoplanet transit, WASP-28b – A previously known hot Jupiter type planet. I’ve posted an image of the transit, it looks beautiful. The noise may seem a little high but this is because the data were taken at 1-min cadence rather than our more usual 30-min cadence. There is also only one transit so we don’t benefit from overlaying multiple transits.

I’ve also being doing some work on estimating what sizes of planet we are going to be sensitive to. It looks we are going to be able to find Earth-sized planets  orbiting relatively bright G and K type stars and well as around fainter M-dwarfs. This is great because these are the stars we are going to be able to get follow-up ground based observations of. I can’t wait to get some more data on the ground.

25 thoughts on “The first K2 observation of a transiting exoplanet

  1. This is great! Did Kepler only observe this particular planet for that limited amount of time or did Kepler spacecraft had all the photometers running for a while?

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    • Thanks! Very happy we captured this one. WASP-28 is about 1 degree out of our ecliptic plane. There are about 10 known transiting planets that are within 5 degrees of the ecliptic. There are about to be a whole lot more.

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  13. I thought you needed three transits to confirm a planet ,and for true earth like planets ( rather than just size) going around G or K stars your viewing windows of 75 days would be too short ? If you revisit the same field of view at a later date , how far out do you expect to be able to find a planet ?

    • Yes, K2 won’t be sensitive to Earth-sized planet on the habitable zone of G and K type stars. Our observing baseline is too short. We might/could/should/definitely will [delete depending how optimistic a person you are] find planets in the habitable zones of M-stars. However, I think the really exciting thing about K2 is finding rocky planets around bright stars that are amenable to RV followup. This will really help us nail down what it means to be a terrestrial planet.

      • Dear Tom. Congratulations on successfully demonstrating that the K2 process works and good luck with the senior review. We need K2. You mention characterisation of terrestrial planets which i agree is great , but I’m not absolutely clear how this will be done. Can you confirm that you intend to look for single planetary transits in atleast some of your time windows/FOV , producing “KOIs” that will have their existence confirmed with ground based RV immediately or retrospectively through synergy with ESPRESSO from 2016 or transit via CHEOPS from 2017 ? This will give as large an orbital period as possible , yet still give a decent transit depth for bigger terrestrial planets . It will also allow characterisation of planets around the largest possible parent stars with correspondingly large semi major axis ,and pushing any potential HBZ of a subject planet out as far as possible from volatile M stars ,perhaps catching a few late K stars too for an 80 day orbital period?

        • We will be observing the same field for 80 days (Campaign 0, our first full test starts next week!) We will be able to find Earth-sized planet orbiting GKM stars with orbital periods <10 days or so. Longer for smaller stars.
          There will be many small planets that can be followed up by the likes of HARPS (north and south) and HIRES on Keck. With this we can measure densities for rocky planets.

          Finding HZ planets is going to be more tricky because of the longer orbital periods of HZ planets. Any small planets we find in the habitable zone will have an orbital periods of just a few weeks. There is an ongoing debate about whether these planets could in fact are potentially habitable. I don’t know enough about this to comment.

          Following up single planet transit would be very interesting but estimating the orbital period from a single transit is fraught with error. I think there is more to gain from following up 2 and 3 transit source.

  14. Sorry. Well done on K2 and good luck with the review. Fingers crossed. We need those extra 2-3 years of info on bright stars it will provide . ESPRESSO then on line just in time to start characterisations , completed by JWST.

  15. Dear Tom. Thanks for your reply into the transit issue I ask about above .
    If three transits to confirm a hit is the rule, do you possess tables that allow calculation of the all important HBZ ( average or range) orbital periods around any given spectral class star ? This must atleast partly govern planning for any prospective transit mission and terrestrial planet characterisation apart,K2 still covers the hypothetical HBZ for later order M stars .If so , can you point me to them?
    Kind thanks

    • Sorry I didn’t reply sooner (I think I’ve partially touched on these points above.
      We can calculate habitable zones for all types of star. A guy at Penn State named Ravi Kopparapu is doing some great work in this area.

      There are more details of the K2 mission in a paper we recently put online.

  16. Dear Tom ,
    Thanks for the above . I spoke to Ravi , who I already know along with his mentor Jim Kasting and he provided me with a very nice graphic of the HBZ per stellar flux.
    Good luck with the imminent senior review. I’m sure you will be ok, the science /cost benefit makes K2 a no brainer really . Its also essential to use the one exoplanet space telescope asset until the next missions arrive.
    I’ve read the arXiv paper . Congratulations on the elegant and innovative design. I see from the paper that you hope to identify exoplanets on just 2 transits for the smaller M stars with the largest signal to noise ratio. Given you are focusing on nearby, “bright” stars, all of which can presumably have RV confirmation of any exoplanet is there no way you could use a “citizen science” ( to keep costs to a minimum) approach to identify planets with just ONE transit ? A 75 day orbital period , rather than 38 , pushes the HBZ out to M0 domain and a bit beyond into larger more “stable” star territory, and further from all the nasty radiation pumped out by Red dwarfs. Its also a good deal bigger than the 36 days slated for TESS.

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