Unknown artist, Planet Quest,
JPL, Caltech, NASA





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Earth-like planets can be defined
as narrowly as those that Earth-type
plants and animals can live on
without special adaptations, but,
even on Earth, multi-cellular life
has adapted to very different
environments over time.

After finding 400 some gas giants by the end of 2009, astronomers have been hoping to find rocky planets that are small enough in mass and diameter within a star's habitable zone to be Earth-like (Franck et al, 2000; and Kasting et al, 1993). NASA's Kepler Mission has proposed to define the size of an Earth-type planet to be one with between 0.5 and 2.0 times Earth's mass, or one having between 0.8 and 1.3 times Earth's radius or diameter. Finding both mass and radius, moreover, would enable the calculation of density which will hopefully lead astronomers to planets made mostly of silicate rocks, rather than water.

Robert Hurt, JPL,
SSC, Caltech, NASA




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Potentially habitable
planets may be common
around nearby Sol-type
stars in the Milky Way
galaxy, according to
one 2008 study (more).

On February 17, 2008, at the Annual Meeting of the American Association for the Advancement of Science, astronomers working with NASA's Spitzer Space Telescope announced that terrestrial planets might form around many, if not most, of the nearby Sun-like stars in the Milky Way galaxy. After surveying six sets of stars with masses comparable to our Sun, Sol, which were grouped by age, they found that planets capable of hosting Earth-type life might be more common than previously estimated. After finding warm dust possibly indicative of terrestrial planet formation in the inner region of nearby, Sol-type stars that are less than 300 million years old, they estimated that at least 20 percent, and possibly as much as 60 percent, of stars similar to the Sun are good candidates for forming rocky planets (JPL (press release; and Meyer et al, 2008). Combined with another study of warm dust from terrestrial planet formation around stars from 10 to 30 million years old (Currie et al, 2008), the two studies suggest that the conditions that led to the formation of Earth may be found around many stars that are between three million and 300 million years old.

Seager et al, 2007,
GSFC, NASA




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Many different types of
low-mass planets may
be possible (more).

Surprises, of course, may recur. Some theorists now believe that some star systems may be slightly to greatly enriched in particular elements in proportions different enough from those found in the Solar System to have formed planets that are quite unfamiliar. For example, some may be slightly richer in carbon rather than oxygen, so that carbon compounds like silicon carbide can be a more common constituent of low-mass planetary bodies than oxygen-rich compounds like silicon dioxide, and this may have even occurred in the outer environs of Sol's nebula so that even Jupiter may have formed from the seed of a "carbon planet" (Kuchner and Seager, 2005; and Katherina Lodders, 2004). Several other planet types, including "iron planets," have also been modelled (NASA GFSFC news feature; Seager et al, 2007; and David Shiga, New Scientist, September 24, 2007). Some of these types of planets may be "second-generation" objects, formed after a star (or companion star) evolves off the main sequence and ejects dust and gas in a planetary nebula or a supernova explosion (Hagai B. Perets, 2010).

Messenger, NASA




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Differences in the composition of
extra-Solar nebulae (i.e., the relative
proportion of elements available for
for processing into crustal minerals
on planetary surfaces) will also
affect habitability for Earth-type life.

Verifying that an Earth-size Planet is Earth-like

Kepler Mission, NASA -- alternative chart by James Kasting

Once an Earth-size planet has been detected in a star's habitable zone, astronomers and planetary scientists would need to ascertain that the planet was actually "Earth-like," habitable for sustaining carbon-based life as is presently known on Earth (Heap and Kuchner, 2009). One way of verifying its similarity to Earth would be to study visible and near infrared light for biological signs of Earth-type life from the planet's host star that was reflected off its atmosphere and surface. According to geoscientist James Kasting who worked on a NASA preliminary design study (that was not completed), the agency's two, complementary but indefinitely postponed Terrestrial Planet Finder (TPF) projects would have been able to detect molecular oxygen (O2), ozone (O3), water vapor (H2O), and possibly even the presence of chlorophyll from photosynthetic lifeforms on a distant planet's surface (James Kasting, 2010). The ESA's similarly delayed Darwin Mission was a similar proposal for confirming the detection of an Earth-like planet. On February 3, 2010, on the other hand, astronomers confirmed the detection of methane (CH4) -- which can be produced by some bacteria -- in the atmosphere of an extra-Solar planet (a close-orbiting "hot Jupiter" found around HD 189733 with a variation of observing planetary transits known as the secondary eclipse method) using only a ground-based telescope, NASA's Infrared Telescope Facility (Swain et al, 2010; Jason Palmer, BBC News, February 3, 2010; and John Matson, Scientific American, February 3, 2010). Kasting, however, notes in his 2010 book "How to Find a Habitable Planet" (page 235) that Earth-size planets around a Sun-like star would be much more difficult to observe using the secondary eclipse method.

Medialab, NASA
© ESA 2001


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To find life on possibly Earth-like
planets around nearby stars, NASA's
and the ESA's indefinitely postponed,
TPF-C and Darwin missions would have
looked for traces of water, oxygen,
and carbon dioxide (CO2) in the
atmospheres of Earth-size planets found
in extra-Solar habitable zones (more).

Messenger, NASA



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With a bit of luck, the Kepler Mission
may find an Earth-sized planet orbiting
a star's habitable zone, within the next
few years.