ness (using the photometer) of 150,000 stars every 30 minutes, searching for tiny dips in the light output that occurs whenever a potential planet passes or “transits” in front of its star. This is called the “transit method” and is Kepler’s principal method in finding planets. Depending on the planet’s orbit and the type of star it orbits, this effect can last anywhere between an hour to about half a day. [4] Transits are only seen when a star’s planetary system is perfectly aligned with our line of sight explained Dr. Howell, “so if you believe that all the orbits are all randomly distributed, as it should be, then Kepler – even if every star had a planet – would only see 1% of those stars having transits.” Regardless, the data received from the spacecraft is extensive in its own merit. More than 13,000 transit-like signals were analyzed and potential new planets were identified [5]. Since not all variations in brightness necessarily represent a transit of a potential planet, there exist false positives, for example stars much like our Sun can vary in brightness themselves. Such temporary phenomenon include ‘Sunspots’ which create visible dark spots caused by intense magnetic activity [6]. For that reason, the discovery of a planet is confirmed by observing a minimum of three transits. But why do three transits constitute a candidate planet? Dr. Alan Gould, co-investigator for the Kepler Mission, explained the need for three transits through an email interview with Technophilic: This would mean planets that are Earth-like and orbit around a star like our sun (every year) would take at least 3 years to get the three transits needed to be confirmed by Kepler to be a candidate planet. Once the planet candidate has been discovered, it is then given the designation of KOI (Kepler Object of Interest). In terms of our new Super-Earth candidate KOI-172.02, it was the 172nd candidate in their running list of candidates to see if it really is a planet and has the right kind of star. For the KOI-172.02 candidate in particular, the 4 transit signals acquired by Kepler indicate that the planet orbits its star around every 243 days. We also know a lot about the star which KOI-172.02 orbits, which is very similar to our sun, but WIRED.CO.UK slightly smaller and colder.
“Three transits are required for planet discovery by the transit method mainly because that is the minimum to assure that there is in fact a planet. One transit gives only the barest indication that a planet exists and an extremely rough idea at best of what the period of the planet might be. Two transits would pinpoint the period of the planet pretty precisely, by virtue of the time between transits, and allow accurate prediction of when the next transit is expected to occur. Actual observation of the 3rd transit confirms the prediction and hence helps confirm the planet discovery.”
The nominal mission of Kepler was 3.5 years, ending last October and is currently in what NASA calls the extended mission – which will last for another 2 or 3 years. “But I think in the next couple of years, Kepler will start providing many more planets around stars like the sun that are much more like the earth”, says Dr. Howell ■
REFERENCES: 1. kepler.nasa.gov/Mission/QuickGuide/ 2. kepler.nasa.gov/Mission/discoveries/ 3. spaceanswers.com/astronomy/370/earth-2-0/ 4. kepler.nasa.gov/Mission/discoveries/fop/ 5. cbc.ca/news/technology/story/2013/01/11/earth-exoplanet-nasa.html 6. solarscience.msfc.nasa.gov/feature1.shtml