Researchers, scientists, and astronomers -- both amateur and professional -- have added hundreds of planets to the rolls of confirmed worlds orbiting other stars in the past few years, and the number is accelerating due to interest and thousands of sighted possible planets revealed by the various telescopes scanning the heavens.
But of all the hundreds of confirmed extra-solar planets thus far, not one of them had been found to be of Earth's size or smaller. This week, scientists announced in Nature that two such planets had been found, one of Earth's relative size and one somewhat smaller, and that those two Earth-sized planets were orbiting a Sun-like star as well.
Dr. Francois Fressin, of the Harvard-Smithsonian Centre for Astrophysics, led the study that found the two worlds. Noting that the discovery touched off a "new era" in extra-solar planet exploration, according to BBC News , Fressin noted that the worlds were found circling Keppler-20, a star that had previously been found to be the center of a planetary cluster of three planets.
The two planets, Keppler-20e and -20f, are believed to be rocky entities, much like the inner planets of our own Solar System. The outer world, -20f, is believed to perhaps once have had an atmosphere. Both were once further from their star and have gradually been drawn inward, the scientists believe.
"We know that these two planets may have migrated closer to their Sun," Fressin told BBC News. "(The larger of the two) might have been an Earth twin in the past. It has the same size as Earth and in the past it could have had the same temperature."
Keppler-20e is smaller than Earth, measuring at 0.87 of Earth's radius. The larger exoplanet is nearly the same size as earth and measures 1.03 of Earth's radius, making it just a bit bigger than Earth. Both lie outside the habitable zone -- and too close to their parent star -- that scientists believe is the most conducive to sustaining life as it is generally understood to exist.
What makes the Fressin, et. al., significant is that it marks the first discoveries of near-Earth-size planets. All other planets detected prior to the study had been fairly larger than Earth. In fact, according to the study, before the detection of Keppler-20e and -20f, the smallest extrasolar planet found had measured 1.42 times the size of Earth.
The search for Earth-like and Earth-sized planets has been ongoing from the start. But the detection of such small worlds is not as simple as it might seem. Although planets are found by studying the gravitational "wobble" of the parent star (a "wobble" generally means that something of mass is affecting the star's orbit and/or rotation), they are often confirmed via their transit shadow on their parent star. Relative size is determined in this manner as well.
Fressin was part of the team that announced in early December that they had found an Earth twin. The BBC reported that, although the planet, Keppler-22b, was 2.4 times the size of our own planet, it was found within the "Goldilocks" or habitable zone. Keppler 22b is about 15 percent closer to its sun than our planet and has a higher average temperature (22 Celsius as opposed to the 14-15 Celsius enjoyed by Earth).
The announcement was accompanied by news at the first Keppler Conference that the Keppler telescope had identified another 2,300 candidate planets. Of those, 207 were believed to be near Earth's size.
Fressin noted that the new discovery is possibly more important than that of Keppler-22b.
"With every new discovery we're getting closer to the 'holy grail' of an Earth-like planet around a Sun-like star," he said. That "holy grail" would be a planet that was similar in size, had a like composition, had a similar atmosphere, and orbited at the relative same distance from the sun -- a true Earth twin.
Prior to the Keppler-22b discovery, scientists had announced the finding of several super-Earths. The first, Gliese-581d, was announced in May. Then HD 85512 b was revealed in August.
Thus far, over 700 planets have been confirmed to exist, according to MSNBC. Of that number, only a small fraction have been found to be in the Goldilocks Zone of habitability. And now, two, one Earth-size and one smaller, can be added to the growing number of detected worlds, although not as candidates of supporting life conditions that are Earth-like.
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