A new “Super Earth” discovered by NASA

A new “Super Earth” discovered by NASA

Project scientists at the Kepler Space Mission have found a total of 461 new planet candidates as of 2013 and one in particular, KOI-172.02, has been described as the most similar to our home planet yet.

KOI-172.02, which stands for Kepler Object of Interest, is a super Earth-size planet candidate, 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 says Dr. Steve Howell, Deputy Project Scientist of the Kepler Mission at Ames.

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 [1]. The habitable zone, as Dr. Howell describes, is the region around a star where water might exist on the surface of a planet which provide favorable conditions for life. The Kepler Spacecraft and photometer, used to observe the stars, orbits the sun each year trailing behind the Earth. As of late, the Kepler Spacecraft has found over 2,500 potential planets [2].

The Kepler mission is designed to detect orbiting planets as they pass in front of their stars, causing a small decrease in the star’s brightness. Kepler does this by looking at just one large region of the Milky Way in the constellations Cygnus and Lyra [3]. 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”, says Dr. Howell.


Over the course of the mission, the spacecraft will measure 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. 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:

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:

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.

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 slightly smaller and colder.

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.


  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

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