KOI-172.02, which stands for Kepler Object of Interest, is a super Earth-size planet, meaning it has a radius 1.5-2 times the size of the Earth. While that may seem insignificant, it means that its mass is much more than that of the Earth, resulting in different properties such as a thicker gaseous atmosphere. It has been described as the most similar to our home planet yet.
The Kepler Mission, launched by NASA in March 2009, was specifically designed to survey a portion in our region of the Milky Way Galaxy to discover Earth-sized planets near the habitable zone, and to determine realistically how many of the billions of stars in our galaxy have such planets. The habitable zone is the region around a star where water might exist on the surface of a planet which provide favorable conditions for life.
The mission is designed to detect orbiting planets as they pass in front of their stars, causing a small decrease in the star’s brightness.
The Kepler Spacecraft and photometer, used to observe the stars, orbits the sun each year trailing behind the Earth. This spacecraft has found over 2,500 planets.
Kepler found planets by looking at just one large region of the Milky Way in the constellations Lyra and Cygnus. This region of space was picked due to certain limiting constraints; an environment rich in stars as well as one that can be continuously viewed and monitored throughout the mission “without obstruction of the Sun to the regions at any point of the spacecraft’s orbit”, say Dr. Howell, Deputy Project Scientist of the Kepler Mission at Ames.
Over the course of the mission, the Kepler spacecraft measures the variations in brightness, 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. 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.
Transits are only seen when a star’s planetary system is perfectly aligned with our line of sight, so if all the orbits are randomly distributed, as it should be, then Kepler – even if every star had a planet – would only see 1% of those stars having transits.
This is called “transit method” and is Kepler’s principal method in finding planets.
Regardless, the data received from the spacecraft is extensive in its own merit. Dozens of thousands of transit-like signals were analyzed and potential new planets were identified. Since not all variations in brightness necessarily represent a transit of a potential planet, there exist false positives. For example, there exist stars much like our Sun which can vary in brightness themselves. Such temporary phenomenon include “Sunspots” which create visible dark spots caused by intense magnetic activity. For that reason, the discovery of a planet is confirmed by observing a minimum of three transits.
Why three transits constitute a candidate planet? According to Dr. Alan Gould, co-investigator of the Kepler mission, the need for three transits explains as follows: 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 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 third transit confirms the prediction and hence helps confirm the planet discovery”.
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 observed, it is then given the designation of KOI – Kepler Object of Interest). In terms of this 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 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 sum, but slightly smaller and colder.
The nominal mission of Kepler was 3.5 years, ending October 2012. Then it was in what NASA calls the extended mission. The next couple of years, Kepler started providing many more planets around stars like the sun that are much more like the Earth.
We have begun to contemplate our origins. Our loyalties are to the species and the planet. We speak for Earth. Image : © Megan Jorgensen