David A. Aguilar, CfA,





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A March 2011 study estimates that
1.4 to 2.7 percent of the Sun-like
stars (of spectral classes FGK) in
the 155,500 sample being observed
by NASA's Kepler Mission may
host Earth- or Super-Earth sized
planets in habitable orbits (more).

Distance from Sol (ly)	Star Type	Planet Name	Mass (<2.0Earths)	Radius/ Diameter (<=1.25Earth)	Orbital Distance (a=AUs)	Habitable Zone (HZ=AUs)	Orbital Period (P=days)	Orbital Period (P=years)	Orbital Eccen- tricity (e)	Equi. Surface Temp. (°F)	Equi. Surface Temp. (°C)
22.7	M1.5-2.5 V	Gl 667 Ch?	=>1.1	~1	~0.09	0.1-0.3	16.9	0.046	0.06	hot?	hot?
300 - 600	M V?	KOI 1422.02	?	0.9	0.12	...	19.9	0.054	?	161	73
400-550	K-M V	Kepler 438 b	>1?	1.12	0.17	...	35.2	0.096	0-0.13	hot?	hot?
490	M V	Kepler 186 f	~1?	~1.1	0.36	0.1?-0.4?	130	0.36	?	warm?	warm?
?	K V?	KOI-1026.01	?	1.77 +/- 0.62	0.33	...	94.1	0.258	?	-24	-31
?	K V?	KOI-701.03	?	1.73 +/- 0.61	0.45	...	122.4	0.335	?	12	-11
~1,750	K-M ?	KOI 736.01	...	...	...	...	...	...	...	55	13
?	K V?	KOI-268.01	?	1.75 +/- 0.61	0.41	...	110.4	0.302	?	71	22
...	...	...	...	...	...	...	...	...	...	...	...
~100?	M?	KOI-326.01	?	0.85 +/- 0.30?	0.05?	...	9.0?	0.025?	?	138?	59?

On February 2, 2011, the Kepler Mission revealed the detection of 54 potential planetary candidates which orbit their host star within or near its apparent habitable zone -- where liquid water can exist on the surface of an Earth-type planet. Five of these planets are near Earth in size, but they orbit stars that are smaller, dimmer, and more orange-red than our own Sun, Sol. While the other 49 are larger, some astronomers wonder if any of them have moons that could be massive enough to support Earth-type life (NASA/Kepler press release).

Messenger, NASA



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As of February 2, 2011, the Kepler Mission
has found five potentially Earth-sized,
planetary candidates with orbits within, or
near, their host star's habitable zone (more).

The five "Earth-sized" planetary candidates have estimated planetary diameters within range of being Earth-sized (0.5 to 1.25 times Earth's diameter) given error margins ranging from 25 to 35 percent due to the uncertainty in the size of their host star and of the "depth" of the observed transits (decrease in stellar luminosity) across the surface of the star. For each planetary candidate, the equilibrium surface temperatures are derived from "grey-body spheres without atmospheres ... [and] calculations assume a Bond albedo of 0.3, emissivity of 0.9, and a uniform surface temperature ... [with uncertainties of] approximately 22% ... because of uncertainties in the stellar size, mass, and temperature as well as the planetary albedo." Actual planetary surface temperature would likely be higher due to warming by any atmosphere gases that might be present (Borucki et al, 2011, pp. 21-23, Table 6).

Kepler Mission, NASA





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KOI-326.01 is no longer thought
to be Earth-sized and habitable
due to further analysis of its
host star, KIC 9880467 (more.

KOI-326.01, possibly the smallest of the five potentially Earth-sized, planetary candidates found in or near their host stars by NASA's Kepler Mission, is now thought to be larger and possibly hotter (see table Potentially Earth-sized Candidates in or near Habitable Zone" below). Due to an erroneous notation in a reference catalog on the brightness of KOI-326.01's host star (KIC 9880467) relative to a "neighboring" star in Kepler's line of sight, further analysis of KIC 9880467 suggests that either the planet is warmer and larger than previously calculated or the planet actually orbits the other star (KIC 9880470) and is much, much hotter. While more analysis is needed, the Kepler team no longer believes that KOI-236.01 is either Earth-sized or potentially habitable (Andrew Grant, Discover, March 8, 2011).

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, where 0.5 Earth-mass is the minimum needed to hold on to an oxygen atmosphere (Catanzarite and Shao, 2011). 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 or some other composition unlike Earth's.

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.

Tim Pyle, UC Berkeley, JPL, CalTech, NASA


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A recent sample survey of 166 "Sun-like" stars within
80 light-years concluded that nearly one in four (23
percent) may host an "Earth-sized" planet within 0.25
AUs of their host star -- i.e., an orbital distance less
than that of Mercury in the Solar System with an
orbital period of 50 days or less (more).

On October 28, 2010, astronomers working on the NASA-UC Eta-Earth Survey (associated with the University of California at Berkeley; see Andrew Howard's July 2010 presentation) announced that "Earth-sized" planets (with 0.5 to two Earth-masses) are probably common around "Sun-like" (i.e., Sol-type) stars. The astronomers used the Keck Observatory for five years to search around a random sample of 166 Sun-like stars of spectral types G, K, and M from 235 of a much larger population of such stars located within 25 parsecs (81.5 light-years) of suitable Hipparcos Catalogue brightness, luminosity, and low chromospheric activity for planets orbiting their host star within only one fourth of an astronomical unit (0.25 AU), which is less than that of Mercury's average orbital distance of 0.4 AU in the Solar System and the equivalent of an orbital period (or "year") of only 50 days or less. Using radial-velocity measurements and the doppler method to detect a "wobble" of about one meter per second, they looked for planets ranging from 1,000 to as little as three times the mass of Earth that they are now able to detect with that method, and then they extrapolated downward for smaller planets. Finding 33 such close-orbiting planets around 22 stars (with 12 planets around 10 more stars still to be confirmed) from the random sample of 166 Sun-like stars, the astronomers determined that many more small planets were found than large ones, which suggests that small planets are more prevalent in Sol's region of the Milky Way Galaxy, and possibly elsewhere as well.

Unknown artist, NASA


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Astronomers are trying to improve their ability to determine
whether any terrestrial Earth-sized planet discovered is truely
"Earth-like" and so potentially habitable for Earth-type plant
and animal life (see the ESA's new Exoplanet Roadmap).

Within the Solar neighborhood, the astronomers estimate that at least one in four (23 percent) in Sun-like stars -- but possibly higher if 12 more planets are confirmed -- have Earth-sized planets, and that roughly 12 percent are "super-Earths with between two and 10 Earth-masses. These results indicate that potentially habitable planets favorable to Earth-type life -- that is, Earth-sized planets orbiting a little farther out from their host stars in the "habitable zone" where liquid water can exist on the planetary surface -- may be relatively common. Applying statistics derived from this survey to NASA's Kepler Mission which uses the less productive transit method of planet detection (that depends on rare change alignments of their orbit in front of their host star but over a much larger survey population of some 156,000 stars), the astronomers forecast that Kepler should find some 120 to 260 Earth-sized planets orbiting close in to Sun-like stars (NASA press release; UC Berkeley press release; Andrew Moseman, Discover, October 28, 2010; Richard A. Kerr, Science Now, October 28, 2010; Paul Gilster, Centauri Dreams, October 28, 2010; and Howard et al, 2010).

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 or much richer in carbon rather than oxygen (e.g., Wasp-12b, NASA/JPL press release; and Beta Pictoris, NASA news release), 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. 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," but in some extra-Solar systems, such carbon-rich planets may have migrated inwards into closer orbits around their host star (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).

Lynette Cook (Artwork from Extrasolar Planets), FUSE, NASA


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High amounts of carbon dioxide have been found around Beta Pictoris,
which suggests that carbon-rich planets may be forming in the outer
region of the disk that is cold enough to prevent oxygen from "burning
up" vaporized carbon dust, while water-rich planets develop closer to
the star (more).

On June 15, 2010, astronomers working on NASA's Kepler Mission released data on all but 400 of some 156,000 target stars. Some 706 stars from this target list were found to have planetary candidates after the first 43 days of observations, but only the identity and some characteristics of 306 stars with at least one planetary candidate were released, including those of five possible multi-planet systems. The Kepler team is holding back data on some 400 of the target stars that are most likely to have Earth-sized -- with planetary candidates of 1.4 Earth-diameters (radii) or smaller within error margin -- and possibly Earth-like planets for further study, until re-scheduled release in February, 2011 (Kepler news release; Dennis Overbye, New York Times, June 15, 2010; Nancy Atkinson, Universe Today, June 15, 2010; Dan Vergano, USA Today, June 15, 2010; Borucki et al, 2010; and Steffen et al, 2010). As these planetary candidates were observed to have a very orbital period (with only 43 days of observation), however, they should be in tight orbits close to their host star and too hot to support liquid water on their planetary surface -- and so not be potentially habitable for Earth-type life (Arbesman and Laughlin, 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

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.

Kepler Mission, NASA -- alternative chart by James Kasting

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. This was accomplished 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 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).

Before the TPF and Darwin are finally funded, built, and launched, some astronomers anticipate that the first studies of Earth-sized, extra-Solar planets ("exoplanets") in visible light will use photometry or low-resolution spectroscopy. They could do this with new space- and Earth-based telescopes that are becoming big and good enough in light-gathering gathering power to analyze the colors of distant planets. In the meantime, some astronomers used NASA's Deep Impact spacecraft to carry out its Extrasolar Planet Observations and Characterization (EPOCh) mission, which included looking back with color filters with its High Resolution Instrument at the major planets in the Solar System, as well as Earth's Moon and Saturn's largest satellite, Titan (in between its visits to and launch of an impactor into the surface of Comet Temple 1 in 2005 and its flyby of Comet Hartley 2 on November 4, 2010).

EPOXI, UMD, GSFC, JPL,
CalTech, Ball Aerospace, NASA



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with NASA video.


Analyzing the colors of an
extra-Solar planet across
light-years can be usful
for determining some basic
atmospheric and surface
characterics (more).

The astronomers then compared the reflected red, blue, and green light imaged from each planetary body based on the wavelengths of sunlight reflected off their surfaces and atmospheres. They found that the planets can be grouped into distinct regions on a plot of the relative amounts of blue versus and red light reflected off them. While the two largest gas giants Jupiter and Saturn can be found in one corner and the smaller gas or "ice giants" Uranus and Neptune in another, the rocky inner planets Mars, Venus, and Mercury are found in yet another area, while the Earth stands alone. They found that Earth stands out from the other planets because its atmosphere scatters a lot of blue light towards the ultraviolet (Rayleigh scattering) and also does not absorb a lot of light in the infrared due a lower amount of infrared-absorbing gases like methane and ammonia when compared with Jupiter and Saturn. As a result, such a three-color analysis may be useful in preliminary analyses of newly found exoplanets to determine whether they are rocky or gaseous bodies and of their type of atmosphere, a means of pre-screening before the final candidates are given a more thorough investigation to ascertain whether they are truly Earth-like or not (NASA news feature with video; Nancy Atkinson, Universe Today, November 4, 2010; and Crow et al, DPS abstract, 2010).

© Ivan Gonçalves, EPOD, USRA, NASA / ESD
(used with permission)

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Depending on star type, a planet with Earth's atmosphere
in a habitable zone orbit would have a different colored
sky and apparent size of their "Sun" (more).

Depending on a main sequence star's spectral type, even a planet with Earth's atmospheric composition may be colored differently. In general, larger and more massive, main-sequence ("dwarf") stars have hotter surface temperatures than our Sun, Sol, and so they radiate more photons, particularly towards the more energetic, bluish end of the spectrum. As a result of their greater luminosity, Earth-like planets would orbit farther away from hotter dwarf stars to avoid getting scorched, but their skies would still appear bluish due to Rayleigh scattering of abundant bluish photons. Around smaller, less massive and dimmer dwarf stars, however, planets would have to orbit closer in order to sustain a surface temperature that is warm enough to keep water liquid and so the star would appear larger in the sky. In addition, stars with surface temperatures of 3,300 kelvins or lower (spectral type M2.5 or redder) would emit so fewer photons towards the bluish wavelengths compared to Sol that the sky would appear whitish down to reddish to Human eyes (more from Earth Science Picture of the Day). If comparatively more bluish or reddish light reaches a planet's surface than on Earth, photosynthetic plant-type life may may not be greenish in color, because such life will have evolved to different pigments in order to optimize their use of available and so color the appearance of the planet's land surfaces accordingly (more discussion of "alien plants").

Doug Cummings, CalTech,
GSFC, NASA



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Photosynthetic lifeforms under
a bluer and brighter star than the
Sun (such as Procyon A or Sirius A)
may reflect less useful or abundant
red and yellow light, or even an
overabundance of dangerously
energetic blue light (more).

On October 11, 2010, the European Space Agency (ESA) released its "scientific and technological roadmap" for pursuing the "characterization of terrestrial exoplanets." This would include planetary parameters such as their mass, radius, density, internal structure, orbit, atmospheric composition, etc.). The ESA's exoplanet roadmap also discusses the findings and limitations of current planet detection and characterization techniques and what new technologies are needed for better detection and analysis of habitable Earth-like planets in coming years (abstract of ESA Exoplanet Road with a link to the full report in PDF). This roadmap followed an earlier U.S. effort, which established the ExoPlanet Task Force in 2006 to advise the National Science Foundation (NSF) and NASA and published its final report (PDF) in 2008.

Distance from Sol (ly)	Star Type	Planet Name	Mass (Earths)	Radius/ Diameter (Earths)	Orbital Distance (a=AUs)	Habitable Zone (HZ=AUs)	Orbital Period (P=days)	Orbital Period (P=years)	Orbital Eccen- tricity (e)	Mean Surface Temp. (°F)	Mean Surface Temp. (°C)	Oxygen in Air (%)
0.0	G2 V	Earth	1.0	1.0	1.0	0.95-1.37 (<0.8-1.65+)	365.24	1.0	0.017	59	15	21
...	...	...	...	...	...	...	...	...	...	...	...	...