Since the Kepler Planet-hunting spacecraft/telescope became operational just over one month ago, it has been busy making news and discovering astronomical ‘firsts’, such as the newest habitable zone exo-planet, Kepler 22b, reported here on earlier this month. But as exciting as that discovery was, the planet was still more than two and a half times the diameter of the Earth (thus giving it a mass significant;y greater than Earth’s).
Now comes news of exoplanetary discoveries that hits a little closer to home, so to speak. A team of astronomers led by Francois Fressin (from Harvard-Smithsonian Center for Astrophysics) have found two “rocky worlds” — one just slightly bigger than our Earth and the other slightly smaller than Venus — orbiting Kepler 20a, a G class star, 1,000 light years away in the constellation of Lyra (note: the members of alien solar systems are given letter designations, with the parent star being ‘a’, with letter sequences proceeding outward according to planet size/type)
Detected through a technique known as radial velocity (where a slight “tug” on a star’s rotation can indicate the presence of an orbiting mass), the two, near-Earth-size worlds have been designated Kepler 20e and Kepler 20f. To date, they are the smallest planets found outside our solar system.
Perliminary measurements show that 20e has a diameter of 6,900 miles (11,000 km) and 20f a diameter of 8,200 miles (13,190 km) which makes them 0.87 and 1.03 times the Earth, respectively. The team of astronomers determined that these were rock-covered worlds (presumably solid all the was through) and so were able to calculate their masses at 1.7 and three times that of Earth’s.
However, both planets are too close to their parent star and thus too hot to be habitable.
More Breakdown on the Planets and Their Solar System
The orbital period of Kepler-20e is 6.1 days at a distance of 4.7 million miles, and the orbital period of Kepler-20f is 19.6 days at a distance of 10.3 million miles. Due to their close orbits to their parent star, the two planets reach temperatures of 760 Celcius (1,400 degrees Fahrenheit) and 426 C (800 degrees F), respectively.
One of the more intriguing aspects to this discovery is that the two planets are members of a somewhat unusual, alternating solar system: big, little, big, little, big (as one moves out from the star) with bigger, gaseous planets alternating with the smaller rocky ones. This is in contrast to our solar system where rocky worlds (Mercury, Venus, Earth, Mars) are separated from the larger gaseous ones (Jupiter, Saturn, Neptune, etc.). The larger, gaseous planets are referred to as Neptune-like worlds (see top image).
Adittionally, the Kepler 20a solar system is highly compacted compared to ours (and the spacing between them fairly regular), with all five (known) planets occupying a space equivalent to that between our Sun and Mercury.
Although some view this alternation of planets as unusual and speculate that it resulted from an inward “migration” of planets which, through interacting with the planetary (accretion) disc from which they were originally formed, allowed them to maintain their relatively regular spacing.
That said, it is not clear at this point how unusual this solar system is in actuality since the catalog of alien solar systems is quite small. It may be that our solar system of segregated planet types is the exception, not the norm.
The discovery of these Earth-sized planets, and the discovery of Kepler 22b earlier this month, bring us closer to finding the ‘holy grail’ of exoplanets — one that is Earth-sized and within the habitable zone of its parent star. Astronomers are predicting that such a discovery will come with two years — perhaps soon given the pace of Kepler Mission exoplanet discoveries.
Some source material for this post came from the Universe Today article First Earth-Sized Exoplanets Found by Kepler