We’ve had such an amazing response to our participation with BBC Stargazing, with over 1 million classifications completed before the start of the third broadcast. We’ve been truly swamped with classifications to search through (and that’s a good thing!). So what do we do with those 1,084,760+ classifications? So what’s next?
Well, we need to look for candidates. I’m working on that part. We have an algorithm to combine the results from the multiple users that classify each light curve in order to sort through and prioritize Q1 light curves for planet candidates. I’ve been using this pipeline to search for planets with orbits less than 15 days and with radii bigger than 2 Earth radii. I’m nearly done with the paper that summarizes the results from that search and hope to get the final numbers in the next week or so and submit the paper to a scientific journal. More on that to come soon. We’ve turned our focus to other Kepler quarters now. We’ve finished classifying all of Q2 light curves, thanks to your hard work. I’ve been starting to apply a modified version of my Q1 search pipeline to Q2. I’m working on improving it and optimizing it for the Q2 light curves, and applying it to the completed light curves from Q4 and Q3 as well. Additionally we’re scouring Talk for new planet candidates and interesting light curves people are discussing, and adding those to our list of interesting objects.
But our job isn’t finished yet. Just seeing a planet transit-like signal in the light curve, isn’t enough to say that it’s a planet and not something else mimicking it. The largest source of false positives for Kepler light curves is faint background eclipsing binaries that are blended with our target star. An eclipsing binary is a double star system where one of the star transits in front of the other as viewed from Earth. If the stars are well separated, you’ll see a characteristic double dip light curve with an alternating pattern of small dip/big dip. When the star transits in front of it’s companion, you see the big drop from the transiting star blocking out the light from it’s companion, and it when it goes behind it’s companion its own starlight is blocked out so you see the smaller dip (or secondary eclipse). Planets aren’t very bright in the optical wavelengths that Kepler is sensitive to. The light observed overwhelming comes from the star, which is why we don’t typically see a secondary eclipse in the Kepler light curves for transiting planets. So if we see that the repeating transit depths for a planet candidate are of different depths, that’s a good indicator the light curve is probably due to an eclipsing binary. But if the eclipsing binary is faint and its light is blended with that of another star, we might not see the secondary eclipse at all. So we still have to do more work to rule out this possibility.
We have a few other checks that we can do such as looking at the position of the brightest pixel for that star on Kepler’s CCDs during and after the suspected planet transit. If that position shifts that tells us we’re likely seeing a blended eclipsing binary. One more test is to get follow-up observations with other telescopes to try and look closer around the target star we think has a planet. Kepler has blurry eyes compared to bigger ground-based telescopes that have adaptive optics systems which can zoom in closer around the Kepler target star. We can use those observations to look and see if we see a companion orbiting or a faint star that is the source of the transits.
We can also try and get radial velocity measurements for the star and measured the wobble induced by the gravitational pull of the orbiting companion. How big the wobble is, tells us the mass of the orbiting body, which would tell us for certain whether the transit is from a planet or orbiting star. But most of the Kepler stars are too faint and the planet candidates are too small to provide a big enough radial velocity signal we can measure from even the best telescopes in the world. Currently the precision is a few m/s for these radial velocity measurements, and something the mass of the Earth orbiting the Sun would cause a wobble of a few cm/s which would be undetectable. So most of the Kepler and Planet Hunters planet candidates will not be confirmed with radial velocity observations but we can still try for those brightest candidate host stars.
We’re working hard on finding planet candidates and vetting the ones we have including our Stargazing planet candidate. Observing proposals to apply for time on the largest telescopes in the world, the 10-m Keck telescopes and 8-m Gemini Telescopes, are due in the next month or so. We’ll be applying for time to take follow-up our top planet candidates. More to news on all these efforts to follow soon.
While we’re working on this, there is lots more light curves that need to be viewed by human eyeballs, so keep the clicks coming, so we can find even more new and interesting planets.