Exoplanet or Extrasolar planet is simply a planet that is located outside of our Solar System. We had always assumed that there were other planets orbiting 200-400 billions of stars in our Galaxy, however detecting them remained a hopeful dream until the 1990's. The problem with detecting a planet, other than the requirement of extremely powerful telescopes, is their proximity to the host star. When we look at another star system, all we see is the intense light (energy) given off by that star. Thus, we cannot look at the exoplanets in the same way that we look at the planets in our Solar System.
Exoplanetary vision. Credit: NASA
However, by the 1990's scientists devised several ingenious ways of detecting planets orbiting around other stars. Instead of trying to see the planets themselves, what we do is study the relationship between a hypothetical planet and its host star. In using Radial velocity method, for instance, scientists looks for the slightest gravitational pull that a planet exerts on its host star. The transit method is utilized to detect exoplanets that are transiting (orbiting) in front of the star, in its relation to us. When a planet passes right before its star, the light output which we observe is slightly less than when there is nothing in front of it. Direct imaging method is, for now, limited to seeing gigantic and extremely bright planets.
The hunt for exoplanets was on. It intensified about 10 years ago, when ever more Astronomers joined in, spending years of observations to successfully prove an existence of yet another alien world. Most of these planets were hot Jupiter-like gas giants, because they are the easiest to detect. They are interesting to study, but fail to satisfy our curiosity for finding planets that are exactly like Earth. We wondered, in these early exoplanet hunting years, whether we will ever find a twin of our small blue dot (Carl Sagan)
Then everything changed when NASA launched into space the Kepler Telescope in 2009. It was designed specifically to hunt for Earth-like planets in a particular region of the Milkyway galaxy. It utilizes a transit method to search for exoplanets among, approximately, 9 million stars. This is a large enough group of stars to allow us to speculate on the frequency of presence of various types of planets in the entire galaxy. The Kepler, however, is yet to confirm its extrasolar planetary bounty of detected planets, because a solid confirmation requires years of observations. Just like Earth orbits around the Sun in a year, other planets need to pass in front of their stars for observations to be complete and accurate. As of now we know of Kepler's extrasolar planetary candidates, not all of which are going to be confirmed. Many have rushed to use this data for premature conclusions.
The Kepler mission, as of July 26, 2011 has confirmed discovery of 17 exoplanets and 1235 exoplanet candidates. Five of the candidate planets are of Earth-size and are in habitable zones. When we say an Earth-like planet, we mean a planet of similar size, rocky composition, as opposed to gaseous, orbiting in its host stars habitable zone. Essentially, the conditions required for a planet to be able to sustain liquid water on its surface. 49 more planet candidates in habitable zone are larger than Earth, ranging from the so-called Super-Earth size (twice as big) to planetary giants like our local Jupiter. We just have to wait for final confirmations before opening a bottle of champagne. This will take a few more years.
Ground based observations have produced over 550 confirmed exoplanets as of 2011, with an accelerating pace of discoveries from one year to another. One planetary system persists to stand out of all, it is Gliese 581, located just over 20 light-years away from Earth. It appears to share some remarkable similarities with our own system. A habitable zone is an orbital distance from its host star where conditions favorable to liquid water can exist. In our Solar System we have Venus that is just on the edge of habitable zone, being too hot, with a runaway greenhouse effect, Earth right within the habitable zone and Mars on the outer edge of it, being slightly too cold. Gliese 581 system appears to have the same three type of planets, Gliese 581 c is on the inner edge, Gliese 581 g is right in the middle of it and Gliese 581 d.
When scientists announced in September of 2010 their discovery and confirmation of the existence of Gliese 581 g planet, it became the first Earth-like world known to us besides our own planet. It is a fascinating, alien world, that is tidally-locked, always facing its star with one side, with the other always remaining in the dark. The terminator line, between both sides, is where Earth-like conditions could exist. Soon, however, controversy was revealed when many in the scientific community concluded that the data was insufficient for a confirmation and the planet was relegated to a candidate status.
Comparison of the Solar System with the Gliese 581 system. The blue line represents the habitable zone. Credit: European Southern Observatory.
This is where the search for Earth-like planets stands today. We have confirmed existence of over 563 planets, including 53 planetary system with at least two planets. The overall scientific consensus, however, is that there is not a single Earth-like confirmed exoplanet yet. The very existence of Gliese 581 g (also known as the Zarmina planet) is put into question, although many are advocating that the conducted observations were sufficient. Either way, it is very likely that NASA's Kepler space-based telescope will confirm an Earth-like exoplanet within a year or two.
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