Updated on June 1, 2013

Habitable Planets

Planets in the Habitable Zone of Stars

Six exoplanets the size of Earth have been discovered in the habitable zones of stars.
NASA's Kepler mission has discovered its first Earth-sized exoplanets candidates for life, meaning they are located in the habitable zone of the star, a region where liquid water can exist on a planet's surface. Five of these planets are approximately the size of Earth, and their orbits are within the habitable zone of stars cooler than our Sun. These life candidates are under observation and monitoring to verify their actual existence. Kepler has confirmed the observation of six planets around a Sun-like star. This is the largest group of transiting planets observed around a single star outside our solar system.
"In one generation, we have gone from science fiction's extraterrestrial planets to reality, and Kepler has made that science fiction into today's reality," said NASA Administrator Charles Bolden. Several hundred new planets have been identified in the Kepler mission data, as published on Tuesday, February 1, 2011. Among the 1,235 planet candidates identified by Kepler at the beginning of 2011, 68 are about the size of Earth, 288 are super-Earths, 662 are Neptune-sized, 165 are Jupiter-sized, and 19 are larger than Jupiter. Fifty-four new planets are in the habitable zone, of which 5 are Earth-sized. The remaining 49 candidates in the habitable zone are super-Earths, ranging from twice the size of Earth to sizes larger than Jupiter. These conclusions are based on observations made between May 12 and September 17, 2009, on over 156,000 stars in Kepler's field of view.

This only covers about 1/400th of the sky.
"This significant number of planets discovered in such a short time and over a tiny fraction of the sky indicates that there must be countless planets orbiting Sun-like stars in our galaxy," said William Borucki of NASA's Ames Research Center in Moffett Field, California.
"We've gone from zero to 68 Earth-sized planets and from zero to 54 candidates in the habitable zone, some of which may have moons and liquid water."
Kepler's results were published in February in the journal Nature. Six planets are orbiting Kepler 11, a Sun-like star located about 2,000 light-years away in the constellation Cygnus. This discovery, thanks to data from NASA's Kepler exoplanet hunter, makes Kepler 11 the most complete exoplanetary system to date. The six inner planets are mixtures of rock and gas, possibly including water. Rocky materials make up the planets' mass, while gas occupies most of their volume. The Kepler space telescope looks for signatures of planets by measuring the slight dimming of starlight caused by planets passing in front of the star (a mini-eclipse).
This is known as a transit. Transits of planets in the habitable zone of Sun-like stars occur about once a year, and three transits are needed to confirm it, so it takes 3 years of observation for Earth-sized planets.
The Kepler science team also uses ground-based telescopes and the Spitzer space telescope to observe candidate planets.

Image: All exoplanets orbiting Kepler-11 are larger than Earth, with sizes comparable to Uranus and Neptune.
The closest planet, Kepler-11b, is ten times closer to its star than Earth is to the Sun.
The other planets are Kepler-11c, Kepler-11d, Kepler-11e, Kepler-11F, and the farthest planet, Kepler-11g, is half as far from its star as Earth is from the Sun.

The Best Place to Live Near the Stars

The circumstellar habitable zone or ecosphere is a theoretical sphere surrounding a star in which the surface temperature of orbiting planets would allow liquid water to be maintained. Scientists believe liquid water is vital because of its role in biochemical reactions. Frank Drake calculated the distance of this zone in his 1961 equation. This equation takes into account the size, luminosity of the star, and the luminosity of our Sun. A star that has 25% of the Sun's luminosity will have a habitable zone centered at approximately 0.50 AU. A star that has twice the Sun's luminosity will have a habitable zone centered at 1.41 AU. A habitable zone does not mean it hosts life but that there is a possibility, based on multiple other factors, that a terrestrial planet could support life: Earth supports life, but the Moon, located in the same zone, is sterile. A planet candidate for the development of life must be massive enough to prevent water from escaping. The habitable zone is not a fixed area; it evolves based on the star's temperature, as stars become brighter and hotter over time, the habitable zone logically moves farther from the star. A planet must therefore remain in this zone as long as possible to develop all the necessary molecules for a form of life.

To determine a planet's habitability, it is not necessary to impose all the conditions that our Earth meets, otherwise no other planet in the Universe could harbor life, as each planet is unique.
Only the condition of having liquid water seems necessary, as it is considered an indispensable element for a viable ecosystem because it greatly facilitates the transport of materials necessary for biochemical activity.
Water is a perfect element for dissolving materials and stores chemical elements very well.
Life can exist outside the habitable zones, it only needs liquid water, a source of energy, and the chemical substances life requires to build its materials.
Astrobiologists believe that certain forms of life could exist on other objects in our solar system, such as Europa, a Galilean moon of Jupiter located 500 million km from Earth.
Europa is an ice-covered sphere the size of our Moon and has an ocean several tens of kilometers deep, kept liquid by the energy from Jupiter's tidal forces. The friction caused by this stretching generates enough heat to keep water liquid beneath the frozen surface.
The discovery of exoplanets began in the 1990s, and by 2011, more than 1,200 exoplanets had been cataloged.