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Larger and jumbo illustrations.
Three, possibly four, super-Earths
have been detected in the habitable
zone around Star C (more), view of
daytime sky from planet "d" as
imagined by Kormmesser.
On June 25, 2013, astronomers announced the Gliese 667 C has at least six planets (possibly seven) and confirmed that at least three (possibly four) super-Earths (possibly four) orbit within the habitable zone around the star (ESO news release; and Anglada-Escude et al, 2013).
On April 18, 2013, astronomers working on NASA's Kepler Mission announced their discovery of two planetary systems that appear to host three super-Earth-size planets (Kepler 62e, 62f, and 69c) in habitable-zone orbits, where the surface temperature of the planet may be suitable for liquid water. Although the Kepler-62 system has five detected planets, the Kepler-69 system has only two thus far. Kepler 62 is a K-2 dwarf star that is smaller, cooler, only a fifth as bright as our Sun, Sol, while Kepler 69 is a Sun-like G-type dwarf with perhaps 93 percent of its mass and 80 percent of its brightness. Kepler-62f is only 40 percent larger in diameter than Earth, making it the extra-Solar planet closest to Earth's size detected in the habitable zone of another star and is likely to have a rocky composition. Similarly, Kepler-62e orbits on the inner edge of its star's habitable zone and is roughly 60 percent larger than Earth. Modelling suggest that both 62e and 62f may be "water worlds" with global oceans. Kepler-69c may be around 70 percent larger than Earth, and its orbit of 242 days around its Sun-like star resembles that of Venus in the Solar System (NASA news release; and Kepler news release).
ARC, JPL, CalTech, NASA
Larger and jumbo illustrations
(from left: Kepler-69c, 62f, 62e, and Earth).
The Kepler Mission has
found three more super-Earths in
in habitable-zone orbits (more).
On February 6, 2013, astronomers analyzing data from NASA's the Kepler Space Telescope announced that some six percent of red dwarf stars may have habitable, Earth-sized planets. Recalibrated for the size of their host star now calculated to be red dwarfs, one Earth-sized and two super-Earth planets were identified as orbiting within their stars' habitable zones. The three habitable-zone planetary candidates identified in this study are: Kepler Object of Interest (KOI) 1422.02, which is 90 percent the size of Earth in a 20-day orbit; KOI 2626.01, 1.4 times the size of Earth in a 38-day orbit; and KOI 854.01, 1.7 times the size of Earth in a 56-day orbit. All three are located about 300 to 600 light-years away and orbit stars with temperatures between 5,700 and 5,900 degrees Fahrenheit (CfA press release; and Dressing and Charbonneau, 2013).
On January 7, 2013, NASA's Kepler Mission announced the detection of 461 new planetary candidates. Four are classed as super-Earths that may orbit within their host star's habitable zone. One (KOI 172.02) only has about 1.5 Earth's diameter and orbits a G-type star somewhat cooler than our Sun, Sol, at a distance of about 0.75 AU with a period of 242 days (Kepler news release).
On December 17, 2012, an astronomer submitted a preprint with new analysis of available radial-velocity data supporting the existence of five planetary candidates around Gliese 667 C. Two planetary candidates were previous detected with orbital periods of 7.2 and 28.1 days ("b" and "c'), while three additional orbital periods of 30.8, 38.8, and 91.3 days ("d," "e," and "f") are also likely to be associated with planetary companions around Gliese 667 C. If confirmed as planets, the 28.1-, 30.8-, and 38.8-day periods would be associated with objects orbiting in the "central portion of the habitable zone, while the 91.3 day orbits lies partly within the habitable zone." The minimum masses for b, c, d, e, and f are 5.4, 4.8, 3.1, 2.4, and 5.4 Earth-masses, respectively. If confirmed, planetary candiate "e" with a 38.8-day period with 2.4 Earth-masses is the lowest mass extra-Solar planet detected in a star's habitable zone to day (Philip C. Gregory, 2012).
On November 7, 2012, a team of astronomers revealed the possible detection of three additional super-Earth-class around this star. Planetary candidates "e," "f," and "g" have a minimum of 3.5 to 7.1 Earth-masses, and their orbits have: average distances of 0.119, 0.247, and 0.600 AU; periods of 34.6, 51.8, and 197.8 days; and eccentricities of 0.15, 0.02, and 0.29, respectively. Orbiting at an average distance within its host star's habitable zone, candidate g has at least 7.1 Earth-masses and so may have a thick atmosphere more like Neptune in the Solar System (Joanna Carver, New Scientist, November 8, 2012); BBC News, November 8, 2012; and Tuomi et al, 2012).
On August 29, 2012, the Planetary Habitability Laboratory (PHL) revealed that a team of astronomers working with the High Accuracy Radial velocity Planet Search (HARPS) project had discovered two planets "b" and "c" around the red dwarf star Gliese 163. Gliese 163 c is a super-Earth with a minimum of 6.9 Earth-masses and a diameter between 1.8 and 2.4 times that of Earth. Located just inside the inner edge of Gliese 163's habitable zone, planet c completes an orbit around its host star in less than 26 days. Receiving on average about 40 percent more light from its host star than Earth does from Sol, planet c has an average surface temperature of about 60°C, compared with Earth's 50°C (PHL press release).
PHL @ UPR Arecibo
Larger and jumbo illustrations.
As of August 29, 2012, the most
habitable super-Earth may be
Gliese 581 "g", among five
other super-Earths near or
within the habitable zone of
their host stars (more).
On July 19, 2012, the Planetary Habitability Laboratory (PHL) announced that new data and analysis suggest that the most habitable super-Earth known on that date may be Gliese 581 g, among four other super-Earths near or within the habitable zone of their host stars. Although the nearby star Gliese 581 is already known for having at least four planets, with Gliese 581 d already suspected of having a habitable orbit, there is now evidence there are two potentially habitable exoplanets orbiting the same star. Both planets are included among the five most habitable found thus far using PHL criteria, including Gliese 667 Cc, Kepler-22 b, and HD 85512 b (PHL press release).
Super-Earth Candidates in or near Habitable Zone
(by equilibrium surface temperature)
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. Although five of these planets are near Earth in size, 16 are candidate super-Earths. While the other 33 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).
The 16 planetary candidates have estimated diameters within range of being super-Earths (1.26 to 2.0 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).
The table below lists Kepler's super-Earth candidates by designation number as a "Kepler Object of Interest" (KOI). This means that the object remains to be confirmed (as opposed to confirmed planets listed as Kepler-number). Also included in the table below is Gl 581 g, which is another super-Earth candidate in its host star's habitable zone whose existence has not been confirmed.
|20.4||M2.5 V||Gl 581 d||5.6-8.4||=>1.5||0.218||0.11-0.28||66.9||0.183||~0||cold?||cold?|
|20.4||M2.5 V||Gl 581 g?||3.1-4.3||1.3-1.5||0.146||0.11-0.28||36.6||0.100||~0||(84)-49||(64)-45|
|22.7||M1.5-2.5 V||Gl 667 Cc||=>3.8||>1<4||~0.12||0.1-0.3||28.1||0.077||0.02||warm?||warm?|
|22.7||M1.5-2.5 V||Gl 667 Cd/f||=>2.7||>1<4||~0.16||0.1-0.3||39.0||0.107||0.03||warm?||warm?|
|22.7||M1.5-2.5 V||Gl 667 Ce||=>2.7||>1<4||0.21||0.1-0.3||62.2||0.170||0.02||warm?||warm?|
|22.7||M1.5-2.5 V||Gl 667 Cf/d||=>5.1||>1<4||0.28||0.1-0.3||91.6||0.251||0.03||cool?||cool?|
|36.4||K5 V||HD 85512 b||~3.6||>1||0.26||0.30-0.59||58.4||0.160||~0.11||warm?||warm?|
|41.8||K2.5 V||HD 40307 g?||=>7.1||>1.5||0.06||0.4-1.0||197.8||0.542||0.29||warm?||warm?|
|48.8||M3.5 V||Gl 163 c||=>6.9||1.8-2.4||~0.1||0.1-0.3||<26||<0.071||?||~140||~60|
|300 - 600||M V?||KOI 854.01||?||1.7||0.24||...||56||0.153||?||-15||-26|
|300 - 600||M V?||KOI 2626.01||?||1.4||0.18||...||38||0.104||?||-46||8|
22 b |
|?||2.38 +/- 0.13||0.85 +/- 0.02||...||289.9||0.794||?||12||-11|
|~1,169||K-M ?||KOI 255.01||?||?||?||...||?||?||?||86||30|
|?||G V?||KOI-1503.01||?||2.68 +/- 0.96||0.54||...||150.2||0.411||?||-24||-31|
|?||?||KOI-947.01||?||2.74 +/- 0.96||0.15||...||28.6||0.078||?||176||80|
|?||?||KOI-817.01||?||2.1 +/- 0.74||0.13||...||24.0||0.066||?||206||97|
|?||?||KOI-1361.01||?||2.2 +/- 0.77||0.24||...||59.9||0.164||?||43||6|
|?||?||KOI-463.01||?||2.2 +/- 0.77||0.24||...||59.9||0.164||?||43||6|
|?||?||KOI-227.01||?||2.2 +/- 0.77||0.24||...||59.9||0.164||?||43||6|
|?||G V?||KOI-536.01||?||2.97 +/- 1.04||0.59||...||162.3||0.444||?||73||23|
|?||K V?||KOI-812.03||?||2.12 +/- 0.74||0.21||...||46.2||0.126||?||82||28|
|?||G V?||KOI-416.02||?||2.82 +/- 0.99||0.38||...||88.2||0.242||?||111||44|
|?||G V?||KOI-555.02||?||2.27 +/- 0.80||0.38||...||86.5||0.237||?||136||58|
|?||G V?||KOI-70.03||?||1.96 +/- 0.67||0.35||...||77.6||0.212||?||140||60|
|?||G V?||KOI-1355.01||?||2.81 +/- 0.98||0.27||...||51.9||0.142||?||156||69|
|?||G V?||KOI-711.03||?||2.62 +/- 0.92||0.49||...||124.5||0.341||?||161||72|
|?||K V?||KOI-174.01||?||2.52 +/- 0.88||0.27||...||56.4||0.154||?||179||82|
|?||G V?||KOI-1564.01||?||3.07 +/- 1.08||0.28||...||53.4||0.146||?||188||87|
|?||G V?||KOI-365.01||?||2.34 +/- 0.82||0.37||...||81.7||0.224||?||194||90|
|?||K V?||KOI-952.03||?||2.4 +/- 0.84||0.12||...||22.8||0.062||?||197||92|
On March 28, 2012, astronomers working with the European Southern Observatory's HARPS instrument announced that super-Earths with habitable-zone orbits may be found in about 41 percent (within a range of 28 to 95 percent) of dim red dwarf (spectral class M) stars within 30 light-years of our Sun. Previously discussed in a November 24, 2011 pre-print, the astronomers "surveyed a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period" and found a "total of nine super-Earths (planets with masses between one and ten times that of Earth)," of which two orbiting within the habitable zones of Gliese 581 and Gliese 667 C. By combining all the radial-velocity data of red dwarf stars (including those without undetected planets) and examining the fraction of confirmed planets that was found, the astronomers were able to estimate the probable distribution of different types of planets around red dwarfs: for example, only 12 percent of such stars within 30 light-years may have giant planets with masses between 100 and 1,000 times that of the Earth (ESO news release; Bonfils et al, 2011; and Delfosse et al, 2011).
Larger and jumbo illustrations.
Kepler 22b is a super-Earth candidate with
around 2.4 times Earth's diameter that orbits
a G5 star within its habitable zone (more).
On December 5, 2011, astronomers working on the Kepler Mission announced their first confirmation an extra-Solar, super-Earth-sized planet orbiting within the "habitable zone" of a distant Sun-like star (spectral class G5), which was discovered using Kepler. Located around 620 light-years away around a star somewhat smaller, cooler, and roughly 25 percent dimmer than Sol which was designated "Kepler-22," planet "b" is estimated to have about 2.38 (+/- 0.13) times Earth's diameter and has an orbital period around Kepler 22 of about 290 days (with an average orbital distance of around 0.85 +/- 0.02AUs). Scientists have not yet been able to determine whether Kepler-22 b has a predominantly rocky, gaseous, or liquid composition (NASA news release; Kepler briefing and discoveries; and science news).
Kepler Mission, NASA
Larger and jumbo illustrations;
and more detailed, close-up map.
As of February 2, 2011, NASA's
Kepler Mission has identified
1,235 planetary candidates based
on more than four months of
observations, of which many are
in 170 multi-planet systems and
54 may orbit within the habitable
zone of their host stars (more).
On Wednesday, February 2, 2011, NASA's Kepler Mission revealed that, thus far, it has detected 1,235 planetary candidates orbiting 907 host stars, from a survey of some 155,453 stars in constellations Cygnus and Lyra using the transit method which requires a rare orbital alignment across the face of the host star as seen from the Solar System. Most of these potential planets appear to be much smaller than gas giants (like Jupiter and Saturn). They include 19 larger than Jupiter and 165 Jupiter-class objects, 662 Neptune-class planets, 288 "super-Earths" (1.26 to 2.0 times Earth's diameter, and 68 Earth-sized planets (0.5 to 1.25 times Earth's diameter) (NASA/Kepler press release; Borucki et al, 2011; and Lissauer et al, 2011a).
et al, 2011,
Kepler Mission, NASA
Larger and jumbo presentation slides.
As of February 2, 2011, Kepler's
planetary candidates have been
identified from only some four
and a half months of observation,
and so the orbital periods of
these 1,235 potential planets
are relatively short (more).
Many of these planets have been found to be members of some 170 multi-planet systems (Lissauer et al, 2011a). Kepler's growing haul of planetary candidates was derived from only four and a half months of observations conducted between May 2 and Sept. 17, 2009. By comparison, confirmation of an Earth-sized planet orbiting its host star at a distance of one AU like the Earth would take a full 12 months of observations, plus another year or two of repeated observation to confirm the orbital period. Lastly, the largest known planetary system other than the Solar System was confirmed to have at least six planets, which have tightly packed orbits around an eight-billion-year-old star located some 2,000 light-years away and include four super-Earths: Kepler-11b, "-11d, "-11e, and "-11f (Kepler news release; Borucki et al, 2011; Lissauer et al, 2011b; and Rachel Courtland, New Scientist, February 3, 2011 -- whose planetary and orbital characteristics are summarized below).
An Evolving Concept
Fellow astronomers were quite surprised in 1990 when Alex Wolszczan and 'Dale Frail announced the discovery of the first three planets outside the Solar System around PSR B1257+12 during a pulsars survey (Wolszczan and Frail, 1992. Given then theories of planetary formation, they had expected to find planets in orbit around "normal" main sequence stars like our Sun, Sol, instead of orbiting stellar remnants like neutron stars which had undergone supernovae. That two of the first three planets were larger than Earth but smaller than the Solar System's four outer gas giants was also unexpected. Not until 2005 did astronomers finally announce the finding of a similarly sized planet around a main sequence star, with the discovery of Gliese (Gl) 876 d.
Larger and jumbo illustrations
(more images and videos).
Gl 876 d may be rocky and have
an atmosphere with clouds and the
glow of molten areas on its surface,
as imagined by Schindler (more).
Using a lower bound of two Earth-masses, astronomers have been increasingly relying on the label "super-Earth" for extra-Solar planets that are probably too large to be very "Earth-like," despite their search for planets with characteristics closer to the Solar System's four rocky inner, "terrrestrial" planets than gas giants. In addition, they have also relied on the predictions of core-accretion models of planetary development that require an upper bound of around 10 Earth-masses to avoid forming large planets that form a significant hydrogen-helium atmosphere like the gas giants (beginning with Uranus in the Solar System). As additional super-Earths are discovered, researchers has been debating whether additional criteria (e.g., planetary radius or density) should be added for making additional distinctions (Seager et al, 2007).
NASA's Kepler Mission originally 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. The mission is also focusing on larger terrestrial planets that have two to 10 Earth-masses, or 1.4 to 2.2 times its radius. While larger planets could have sufficient gravity to attract a massive hydrogen-helium atmosphere, smaller planets -- like Mars or Mercury that have less than half the Earth's mass -- located in or near their star's habitable zone may lose their initial life-supporting atmosphere because of low gravity and/or the lack of plate tectonics needed to recycle heat-retaining carbon dioxide gas back into the atmosphere (Kasting et al, 1993). In contrast, super-Earths with a similar concentration but larger absolute amount of radioactive heat sources (i.e., uranium and thorium) than Earth would produce more internal heat, more vigorous mantle convection, and faster plate tectonic action involving thinner plates, which may promote planetary habitability with lower mountain ranges but higher volcanic activity and an atmosphere with a greater relative composition of volcanic and lighter gases (Sasselov and Valencia, Scientific American, August 2010; Valencia and O'Connell, 2009; and Valencia et al, 2007).
Assuming an iron-rich planet with an internal structure like Earth, modelling results for the first discovered super-Earth (GJ 876 d) indicate the existence of a threshold in planetary diameter above which a super-Earth "most certainly" has a high water content (an "ocean planet" or "water world," where thick layers of water and pressurized ice surround a rocky mantle and core); this threshold was found to be around 24,000 kilometers (or nearly 15,000 miles) in the particular case of GJ 876 d (Valencia et al, 2007). Given the same mass, ocean planets are around 40 to 50 percent larger than rocky planets (Fortney et al, 2007). Water worlds, however, may be most likely type of super-Earth to be habitable for photosynthesis-based Earth-type life (von Bloh et al, 2009).
NASA -- larger image
Many charged particles from the Sun's Solar Wind are trapped by Earth's magnetic field
to form its magnetosphere, deflecting the Wind from a head-on collision with its atmosphere.
In 2010, model simulations of rocky super-Earths between two and 10 Earth-masses indicated that high pressures could keep their cores solid instead of molten, which would prevent a protective magnetic field from forming protecting developing surface life from stellar radiation. Without a magnetic field generated by a rotating molten metallic core, the atmosphere of such a planet would also face progressive erosion by the stellar wind of its host star. Other scientists caution that the interiors of super-Earth may still get hot enough to melt their iron cores despite the pressure due to other factors not yet considered by the model simulation (New Scientist, November 6, 2010; and Morard et al, 2010).
Super-Earths within 100 light-years
15.2 M4 V Gl 876 d 5.9-7.5 1.7 0.021 0.11-0.22 1.9 0.0053 0.10 315-711 157-377
19.8 G8 V 82 Eridani b =>2.68 >1 0.12 0.56-1.11 ~18 0.05 0.? hot hot
19.8 G8 V 82 Eridani c =>2.38 >1 0.20 0.56-1.11 ~40 0.11 0.? hot hot
19.8 G8 V 82 Eridani d =>4.72 >1 0.35 0.56-1.11 ~90 0.25 0.? hot hot
20.4 M2.5 V Gl 581 c =>5.6 =>1.5 0.07 0.11-0.28 12.9 0.035 ~0 hot hot
20.4 M2.5 V Gl 581 d 5.6-8.4 =>1.5 0.22 0.11-0.28 66.9 0.183 ~0 cold? cold?
20.4 M2.5 V Gl 581 e 1.7-3.1 >1 0.03 0.11-0.28 3.1 0.085 ~0 hot hot
20.4 M2.5 V Gl 581 f =>7.0 >1.5 0.758 0.11-0.28 433 1.19 ~0 frigid frigid
20.4 M2.5 V Gl 581 g? 3.1-4.3 1.3-1.5 0.146 0.11-0.28 36.6 0.100 ~0 (29)-160 (34)-71
Gl 667 Cb =>5.7 >1 ~0.05 ~0.1-0.3+ ~7 ~0.02 ? hot hot
Gl 667 Cc? =>3.4 >1<4 ~0.12 0.1-0.3 28.1 0.077 ~0 warm? warm?
27.8 G5-6 V 61 Vir b =>5.1 >1 0.050 0.8-1.5 4.2 0.011 0.12 hot hot
29.5 M1.5 V Gl 433 b =>6.0 >1 0.056 0.18-0.35+ ~7 ~0.02 ? hot hot
30.7 M2.5 V Gl 176 b =>8.4 >1 0.066 0.11-0.28? 8.8 0.023 ~0 hot hot
36.4 K5 V HD 85512 b ~3.6 >1 0.26 0.30-0.59 58.4 0.160 ~0.11 warm? warm?
40.9 G8-K0 V 55 Cancri e 8.6 1.6 0.016 0.7-1.4 0.7 0.002 0.07 3,300-4,900 1,800-2,700
41.0 G7.5-K0 V HD 69830 b =>10.5 >1 0.078 0.6-1.3 8.7 0.024 0.10 =>530 =>990
41.8 K2.5-3 V HD 40307 b =>4.2 1-1.5 0.055 0.44-0.87 4.3 0.012 ~0 hot hot
41.8 K2.5-3 V HD 40307 c =>6.7 1-2 0.082 0.44-0.87 9.6 0.026 ~0 hot hot
41.8 K2.5-3 V HD 40307 d =>9.4 ~2 0.136 0.44-0.87 20.4 0.056 ~0 hot hot
42.1 M4.5 V GJ 1214 b 5.57-7.53 2.53-2.74 0.014 ~0.1-0.2 1.6 0.0044 <0.27 >540 >280
54.0 K0V HD 7924 b =>9.3 >1 0.057 0.5-1.1 5.4 0.015 ~0.2 hot hot
69.5 K1 V HD 97658 b =>8.2 >1 0.083 0.5-1.0 9.5 0.026 ~0 hot hot
76.4 G0 V HD 1461 b =>6.94 >1 0.063 0.8-1.6 5.8 0.016 ~0 hot hot
76.4 G0 V HD 1461 c =>5.92 >1 0.112 0.8-1.6 13.5 0.037 ~0 hot hot
78.4 K2 V HD 156668 b =>4.2 >1 ~0.05 0.47-0.93 4.6 0.013 ~0 hot hot
~85 K3-5 V-III HD 181433 b =>7.5 1-2 0.08 0.65-0.87 9.5 0.026 0.40 hot hot
... ... ... ... ... ... ... ... ... ... ... ...
On January 26, 2012, scientists working on NASA's Kepler Mission team announced the discovery of 11 new planetary systems hosting 26 confirmed planets, as well as additional planetary candidates. These discoveries nearly double the number of confirmed planets discovered using the Kepler space telescope and triple the number of stars known to have more than one planet that "transits" in front of its host star. Fifteen of the newly confirmed planets are estimated to be only between Earth and Neptune in size, and the smallest may have a diameter only 50 percent larger than Earth's (Kepler 33b). The smallest planet orbits Kepler-33, a star older and more massive than our Sun, Sol, which also had the most detected planet candidates at five (ranging in size from 1.5 to 5 times that of Earth) in uninhabitable, hot inner orbits closer to their star than even Mercury around our Sun (NASA Kepler news release; and JPL news release).
On January 11, 2012, astronomers working with the European Southern Observatory (ESO) announced that most stars in our Milky Way Galaxy have planets. They used gravitational microlensing to analyze how common planets may be around stars. After six years of observation involving millions of stars, they concluded "that stars are orbited by planets as a rule, rather than the exception." While other techniques are biased towards detecting planets close to their stars (such that 17 to 30 percent of Sol-type stars have been found to have such inner-orbit planets), gravitional lensing has found reveals the fraction of planets at farther orbits. In their survey of planets within 0.5 to 10 AUs of their host star, the astronomers found that 17 +6/-9 percent of observed stars had Jupiter-class planets (of 0.3 to 10 Jupiter masses), 52 +22/-29 percent had Neptune-class planets (of 10 to 30 Earth-masses), and that 62 +35/-37 percent had super-Earths of 5 to 10 Earth-masses, which is consistent with the conclusion that an average star in the Milky Way should have one or more planets within an orbital distance of 0.5 to 10 AUs, and that there may be some 10 billion Earth-sized planets in the galaxy (ESO press release; Anil Ananthaswamy, New Scientist, January 11, 2012; Jason Palmer, BBC News, January 11, 2012; and Cassan et al, 2012).
Larger and jumbo illustrations.
A planet detection effort using
gravitational microlensing suggests
that most stars in our galaxy have
planets in orbit around them, of
which many are super-Earths (more).
On December 5, 2011, on the opening of its inaugural science conference at NASA's Ames Research Center, the Kepler Mission team announced 1,094 new planetary candidates, bringing the running total of potential planet discoveries to 2,326. Of those Kepler planetary candidates, 207 are roughly Earth-size, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size, and 55 are larger than Jupiter. Based on Kepler observations conducted from May 2009 to September 2010, the planet findings show a dramatic increase in the numbers of smaller-size planet candidates, where Earth-size and super-Earth-size candidates have increased in number by more than 204 and 136 percent, respectively, since the last Kepler announcement in February 2011. The new data indicate that planets from one to four times the size of Earth may be abundant in our galaxy. Thus far, Kepler has found 48 planetary candidates in their host star's habitable zone (of which 10 are near Earth-size), but this number is a decrease from the 54 reported in February 2011 only because the Kepler team is now applying a stricter definition of what constitutes a habitable zone around stars to account for the warming effect of planetary atmospheres, which would move such a zone away from the star, outwards in orbital distance resulting in longer orbital periods (NASA news release; and Kepler Press Conference slides -- in pdf).