The Search for Earth-like Planets
A look at how astronomers search for other Earth-like planets around
other stars; which stars are candidates for possible discovery; and how
techniques develop and the sensitivity of equipment improve will make
finding another Earth just a matter of time.
An Earth analog is a theoretical other planet or world with conditions similar to Earth.
Humans have long speculated on the existence of such a planet and there is considerable interest in the subject expressed in science, philosophy, science fiction and popular culture.
The majority of planets found outside our solar system have been gas giants since they produce more pronounced wobbles in the host stars and are thus more easily detectable. However, a number of extrasolar planets are suspected to be terrestrial.
As the solar system proved to be devoid of an Earth analog, the search has widened to extrasolar planets. The mediocrity principle suggests that there is a chance that some serendipitous events may have happened elsewhere.
However the Rare Earth hypothesis suggests that if the strictest criteria are applied, such a planet, if it exists may be so far between the human race may never know.
It is also thought by some a large natural satellite (or habitable moon) may be similar to Earth. Astrobiologists would assert that Earth analogs would most likely be found in a habitable zone in which liquid water and conditions for supporting life.
During the early 1990s, the first extrasolar planets were discovered orbiting the pulsar PSR B1257+12 with masses of 0.02, 4.3, and 3.9 times that of Earth's. They were discovered by accident: their transit caused interruptions in the pulsar's radio emissions.
When 51 Pegasi b, the first planet found around a star still undergoing fusion, was discovered, many astronomers assumed it must be a gigantic terrestrial, as it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did.
However, subsequent diameter measurements of a similar extrasolar planet (HD 209458 b), which transited its star showed that these objects were indeed gas giants.
Because of the limited techniques currently available to detect extrasolar planets, many of those found to date have been of a size associated, in our solar system, with gas giants.
Because these large planets are inferred to share more in common with Jupiter than with the other gas giant planets, some have claimed that "Jovian planet" is a more accurate term for them.
In June 2005, the first planet around a fusing star that may be terrestrial was found orbiting around the red dwarf star Gliese 876, 15 light years away. That planet, Gliese 876 d, has a mass 7 to 9 times that of earth and an orbital period of just two Earth days.
But the radius and composition of the planet is unknown.
On 10 August 2005, Probing Lensing Anomalies NETwork/Robotic Telescope Network (PLANET/RoboNet) and Optical Gravitational Lensing Experiment (OGLE) observed the signs of a cold planet designated OGLE-2005-BLG-390Lb, about 5.5 times the mass of Earth, orbiting a star about 21,000 light years away in the constellation Scorpius.
The newly discovered planet orbits its parent star at a distance similar to that of our solar system's asteroid belt. The planet revealed its existence through a technique known as gravitational microlensing, currently unique in its capability to detect planets with masses down to that of Earth.
In April 2007, a team of 11 European scientists announced the discovery of a planet outside our solar system that is potentially habitable, with Earth-like temperatures. The planet was discovered by the European Southern Observatory's telescope in La Silla, Chile, which has a special instrument that splits light to find wobbles in different wave lengths.
Those wobbles can reveal the existence of other worlds. What they revealed is planets circling the red dwarf star, Gliese 581. Gliese 581 c was considered to be habitable at first, but more recent study in April 2009, suggests Gliese 581 d is a better candidate.
Regardless, it has increased interest in examining planets circling dimmer stars. About 80 percent of the stars near Earth are red dwarfs. The Gliese 581 (c and d) planets are about five to seven times heavier than Earth, classifying them as super-Earths.
Gliese 581 e is only about 1.9 Earth mass, but could have 2 orders of magnitude more tidal heating than Jupiter’s volcanic satellite Io. An ideal terrestrial planet would be 2 Earth masses with a 25 day orbital period around a M dwarf star.
The discovery of Gliese 581 g was announced in September 2010, and is believed to be the first so-called "Goldilocks planet" ever found, the most Earth-like planet, and the best exoplanet candidate with the potential for sheltering life found to date.
The Kepler Mission endeavours to discover Earth-like planets orbiting around other stars by observing their transits across the star. The Kepler spacecraft was launched on March 6, 2009.
The duration of the mission will need to be about three and a half years long to detect and confirm an Earth-like planet orbiting at an Earth-like distance from the host star. Since it will take intervals of one year for a truly Earth-like planet to transit (cross in front of its star), it will take about four transits for a reliable reading.
The probability of finding an Earth analog depends mostly on the attributes which are expected to be similar, and these vary greatly. Generally it is considered that it would be a terrestrial planet and there have been several scientific studies aimed at finding such planets.
Often implied but not limited to are such criteria as planet size, star size (i.e. simlar to the Sun), orbital distance and stability, axial tilt and rotation, similar geography, oceans, air and weather conditions, magnetic field and even the presence Earth-like complex life (possibly through convergent evolution or parallel evolution).
If there is complex life, there could be some forests covering much of the land. If there is intelligent life, some parts of land could be covered in cities. Some things that are assumed of such a planet may be unlikely due to Earth's own history.
For instance the Earth's atmosphere was not always oxygen-rich and this is a biosignature from the emergence of photosynthetic life. The formation, presence, influence on these characteristics of the moon (such as tidal forces) may also pose a problem in finding an Earth analog.
Dimitar Sasselov: How we found hundreds of Earth-like planets
Dimitar Sasselov, the Kepler mission co-investigator, recently mentioned at the 2010 TED Conference that there have been hundreds more candidate terrestrial planets discovered since Kepler went online.
If these planets are confirmed via further investigation, then it will represent the largest find of extrasolar planets to date.
The Kepler science teams are, for now, keeping the initial results of any candidate planets a secret so they can confirm their results.
On February 2, 2011, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 54 that may be in the "habitable zone." Some of these candidates were "Earth-size" and "super-Earth-size" (defined as "less than or equal to 2 Earth radii".
Six of these candidates are in the "habitable zone."
A more recent study found that one of these candidates is in fact much larger and hotter than first reported. A number of other telescopes capable of directly imaging extrasolar terrestrial planets are also being designed. These include the Terrestrial Planet Finder, Space Interferometry Mission, Darwin, New Worlds Mission, and Overwhelmingly Large Telescope.
The 2011 estimates by the NASA's Jet Propulsion Laboratory (JPL) based on observations from the Kepler mission determined that about "1.4 to 2.7 percent" of all sunlike stars are expected to have earthlike planets "within the habitable zones of their stars".
This means there are "two billion" of them in our own Milky Way galaxy alone and assuming that all galaxies have a similar number as the Milky Way, in the 50 billion known galaxies there may be as many as one sextillion.