Trent Schindler, NSF


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).

Jonathan Langton,
Principia College


Larger simulation slide
of atmospheric flows.


61 Virginis b has an inner
"torch" orbit that should
heat its atmosphere so
much that it glows (more).

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).

© ESO



Larger animation still.



Gliese 581 d orbits within its host
star's habitable zone and so may
have liquid surface water in a deep
global ocean, as a "water world" (more).

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).

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).

Dan C. Fabrycky, U.C. Santa Cruz, Kepler,
Ames, JPL, Caltech, NASA




Larger and jumbo illustration.



Kepler has discovered 11 more
star systems with at least 26
confirmed planets (more).

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).

Martin Kornmesser, ESO



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).

Unknown artist, Kepler, Ames, JPL, Caltech, NASA



Larger and jumbo illustrations.



As Kepler 22 is slight smaller and cooler than
our Sun, Sol, its habitable zone is slightly
smaller and located closer to the star than the
habitable zone around our own Sun (more).

In a pre-print submitted on November 21, 2011, a team of astronomers revealed the finding of a second potential super-Earth "c" with at least 3.4 Earth-masses in a potentially habitable-zone orbit (~0.12 AUs) with a period of 28.1 days around Gliese 667 C, or MLO 4 C, (Bonfils et al, 2011; and Delfosse et al, 2011, in prep).

© Lynette Cook (Artwork from
Extrasolar Planets - Collection III,
used with permission)


Larger and jumbo illustrations.


Like Gliese 581 g, Gliese 667 Cc is
believed to be a super-Earth that orbits
Gliese 667 C within the red dwarf
star's habitable zone (more).

On September 12, 2011, astronomers working with the High Accuracy Radial velocity Planet Searcher (HARPS) at the European Southern Observatory (ESO) announced the discovery of 16 super-Earths (including HD 85512 b discussed below) among 50 newly detected extra-Solar planets (ESO science release; and Mayor et al, 2011).

© ESO



Larger animation still.



Like Gliese 581 d, HD 85512 b orbits just
within its host star's habitable zone and so
may have liquid surface water on its surface
under favorable atmospheric conditions.

On August 17, 2011, astronomers associated with the High Accuracy Radial velocity Planet Searcher (HARPS) at the European Southern Observatory (ESO) uploaded a pre-print that revealed the discovery of a super-Earth of around 3.6 +/- 0.5 Earth-masses in a potentially habitable zone orbit around HD 85512 (also known as CD-42 5678 or Gliese 370) (Pepe et al, 2011; and Kaltenegger et al, 2011). The planet complete an orbit around HD 85512 in 58.43 +/- 0.13 days at an average distance of 0.26 +/- 0.005 AU in an mildly elliptical orbit with an eccentricity e = 0.11 +/- 0.10 (Pepe et al, 2011; and Kaltenegger et al, 2011, Table 1). This orbits places the planet near the inner edge of its host star's habitable zone, where liquid water could exist in liquid form under favorable conditions such as an albedo of 0.52 with an orbital eccentricity of 0.11 and more than 52 percent cloud cover under a sufficiently dense atmosphere of water, carbon dioxide, and molecular nitrogen like Earth's (ESO science release; Pepe et al, 2011; and Kaltenegger et al, 2011 -- more below).

Martin Kornmesser, ESO



Larger and jumbo illustrations (source).



HD 85512 b has some 3.6 Earth-masses
and appears to orbit near the estimated
inner edge of the habitable zone around
its host star, where liquid water, and
possibly life, may exist under favorable
conditions (more).

On June 10, 2011, a team of astronomers submitted a preprint which revealed the detection of a possible cold super-Earth around a low-mass star around 9,900 +/- 1,100 light-years away in Constellation Scorpius (17:48:5.06-35:00:19.8) using the gravitational microlensing method. It was first detected on June 1, 2009 by the Microlensing Observations in Astrophysics (MOA) collaboration with the 1.8-meter telescope at the Mount John University Observatory in New Zealand. Designated MOA-2009-BLG-266L b, the planet has a mass of around 10.4 +/- 1.7 Earth-masses and orbits its host star at a semi-major axis of 3.2 +1.9/-0.5 AUs with an orbital period of 7.6 +7.7/-1.5 years. MOA-2009-BLG-266L, the host star has only some 0.56 +/- 0.09 of a Solar-mass. Measurement corroboration of the planet's mass as well the microlensing parallax were undertaken with the Deep Impact / EPOXI spacecraft, which was in a Heliocentric orbit (around our Sun, Sol). The planet's mass and orbital distance are around the theoretical threshold values needed to accrete a substantial gas envelope of hydrogen and helium like Uranus (Muraki et al, 2011; and the Extrasolar Planet Encyclopaedia entry for MOA-2009-BLG-266L b).

NASA



Larger image of Uranus (source).



A new super-Earth with around 10
Earth-masses (that may have a gaseous
atmospheric envelope of hydrogen and
helium like Uranus) has been detected
in a cold orbit around a dim, low-mass
star aroud 9,900 light-years away (more).

On May 23, 2011, astronomers on the Kepler Mission announced confirmation of another planet in the Kepler-10 system, designated Kepler-10c. It's existence was "validated using a combination of a computer simulation technique called 'Blender' and NASA's Spitzer Space Telescope. Kepler-10c appears to be a super-Earth class planet with 2.2 times Earth's radius, and it orbits the star every 45 days (which is longer than Kepler-10b's eight-day orbit and so must have a longer orbital distance). Although also a super-Earth class planet, Kepler-10b has only 1.4 times Earth's radius and has been determined to be one of the smallest rocky planets detected thus far. Both Kepler-10b and 10c, however, are calculated to be "blistering hot worlds" (Kepler news release; Fressin et al, 2011; and Francois Freesin's PowerPoint presentation slides).

Unknown artist, JPL, SSC, Caltech, NASA


Larger and jumbo illustrations (source).


A new super-Earth "c" has been detected
in the Kepler 10 system around 560
light-years away, which should be a
"blisteringly hot" world (more).

On March 8, 2011, two astronomers analyzing the first four months of data from NASA's Kepler Mission submitted a pre-print with an initial estimate that the 1.4 (+/- 0.5) to 2.7 (+/- 0.9) percent of the Sun-like stars (of FGK spectral types) in the Kepler sample of nearly 155,500 may host Earth- and super-Earth sized planets with orbits within their host star's habitable zone (Catanzarite and Shao, 2011). The variance in the estimates depended on the use of a conservative versus a more conventional definition of habitable zone (Kasting et al, 1993; and ExoPlanet Task Force, 2008 report in pdf, pp. 102-103). Their calculations suggested that Kepler will eventually find a total of 12 such "Earth-analog" planets after three to four years of observation, and that four of these planets have already been detected. The astronomers expect that this estimate of Earth-analog planets can be refined as Kepler's data set grows to 3.5 to six years of observations (Catanzarite and Shao, 2011; and Victoria Jaggard, National Geographic News, March 29, 2011).

David A. Aguilar, CfA,





Larger and jumbo illustrations.





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).

On October 11, 2010, an astronomer from the Geneva Observatory's High Accuracy Radial velocity Planetary Search (HARPS) project announced at the IAU Symposium 276 that their team had not been able to confirm the existence of either planetary candidates Gliese 581 "g" or ""f" based on only their own, expanded but smaller dataset of 180 observations over 6.5 years. Simulations based on their data have shown that the probability that the radial-velocity variations can be produced "by chance" because the noise is not negligible. This is because the signal amplitude of "g" and "f" are very low and close to the level of the measurement noise (Leslie Mullen, Astrobiology, October 12, 2010; and Rachel Courtland, New Scientist, October 13, 2010).

© ESO, unknown artist




Larger illustration.




Using a smaller dataset over 6.5
rather than 11 years, a rival team
of astronomers has not been able
confirm the existence of planetary
candidates "g" and "f" (more).

On September 29, 2010, a team of astronomers announced the discovery of a super-Earth sized, rocky planet orbiting within the habitable zone of nearby, red dwarf star Gliese 581 (also known as HO Librae but first catalogued as BD-07 4003), based on 11 years of radial-velocity measurements at the Keck Observatory and similar published data from the Geneva Observatory's HARPS project. Located towards Constellation Libra, the Gliese 581 system is now believed to have at least six planetary candidates, of which the "potentially habitable" planet has been designated as Gl 581 "g." This planetary candidate is estimated to have between 3.1 and 4.3 Earth-masses and between 1.3 and 1.5 times Earth's diameter, with a greater surface gravity of around 1.1 to 1.7 g.

© Lynette Cook (Artwork from
Extrasolar Planets - Collection III,
used with permission)


Larger and jumbo illustrations.


Planetary candidate "g" is believed
to be a rocky super-Earth that orbits
Gliese 581 within the red dwarf
star's habitable zone (more).

Gliese 581 g has an orbital period of 36.6 days at an average orbital distance of 0.146 AUs. Such a planetary orbit is likely to be in synchronous rotation with its host star, so that one side is always facing the star with perpetual daylight while the other side is in perpetual darkness. As the planet is massive enough to hold on to a thick atmosphere with heat-circulating winds, however, it may host a wide range of relatively stable climates, with the most hospitable region for Earth-type life around the line between shadow and light known as the "terminator." Although the average surface temperature is estimated to be between -24° F (-31° C or 242° Kelvin) and 10° F (-12° C or 261° Kelvin), actual surface temperatures could range from very hot at around 160 °F (71 °C) on the daylight side to very cold at around -29 °F (-34 °C) on the dark side (NASA news release; Carnegie news release; U.C. at Santa Cruz press release; NSF video announcement; David Shiga, New Scientist, September 29, 2010; Dennis Overbye, New York Times, September 29, 2010; Astronomy Picture of the Day; and Vogt et al, 2010).

On June 10, 2011, a team of astronomers submitted a preprint which revealed the detection of a possible cold super-Earth around a low-mass star around 9,900 +/- 1,100 light-years away in Constellation Scorpius (17:48:5.06-35:00:19.8) using the gravitational microlensing method. It was first detected on June 1, 2009 by the Microlensing Observations in Astrophysics (MOA) collaboration with the 1.8-meter telescope at the Mount John University Observatory in New Zealand. Designated MOA-2009-BLG-266L b, the planet has a mass of around 10.4 +/- 1.7 Earth-masses and orbits its host star at a semi-major axis of 3.2 +1.9/-0.5 AUs with an orbital period of 7.6 +7.7/-1.5 years. MOA-2009-BLG-266L, the host star has only some 0.56 +/- 0.09 of a Solar-mass. Measurement corroboration of the planet's mass as well the microlensing parallax were undertaken with the Deep Impact / EPOXI spacecraft, which was in a Heliocentric orbit (around our Sun, Sol). The planet's mass and orbital distance are around the theoretical threshold values needed to accrete a substantial gas envelope of hydrogen and helium like Uranus (Muraki et al, 2011; and the Extrasolar Planet Encyclopaedia entry for MOA-2009-BLG-266L b).

NASA



Larger image of Uranus (source).



A new super-Earth with around 10
Earth-masses (that may have a gaseous
atmospheric envelope of hydrogen and
helium like Uranus) has been detected
in a cold orbit around a dim, low-mass
star aroud 9,900 light-years away (more).

On May 23, 2011, astronomers on the Kepler Mission announced confirmation of another planet in the Kepler-10 system, designated Kepler-10c. It's existence was "validated using a combination of a computer simulation technique called 'Blender' and NASA's Spitzer Space Telescope. Kepler-10c appears to be a super-Earth class planet with 2.2 times Earth's radius, and it orbits the star every 45 days (which is longer than Kepler-10b's eight-day orbit and so must have a longer orbital distance). Although also a super-Earth class planet, Kepler-10b has only 1.4 times Earth's radius and has been determined to be one of the smallest rocky planets detected thus far. Both Kepler-10b and 10c, however, are calculated to be "blistering hot worlds" (Kepler news release; Fressin et al, 2011; and Francois Freesin's PowerPoint presentation slides).

Unknown artist, JPL, SSC, Caltech, NASA


Larger and jumbo illustrations (source).


A new super-Earth "c" has been detected
in the Kepler 10 system around 560
light-years away, which should be a
"blisteringly hot" world (more).

Dana Berry, Kepler, NASA


Larger and jumbo animation stills.


The Kepler Mission has detected
a rocky super-Earth in a torch orbit
around an old star around 560 years
away (more).

On January 10, 2011, astronomers working with NASA's Kepler space telescope announced the discovery of a rocky super-Earth of only 1.4 Earth-diameters across and around 4.6 Earth-masses, at the 217th Annual Meeting of the American Astronomical Society. Designated "Kepler-10b," the planetary candidate was already catalogued as Kepler Object of Interest or (KOI) 72.01. It orbits an aged, spectral type G star (KIC 11904151) of 0.895 Solar-mass and 1.06 Solar-diameter with around 1.00 Solar luminosity, which is estimated to be at least 7.4 billion years old. The Kepler-10 system lies around 560 light-years away in the northwestern corner of Constellation Cygnus.

Kepler Mission, NASA





Larger chart.





Kepler 10b appears to
be a rocky planet that
is probably denser
than Earth (more).

With a torch orbit around its host star that takes only about 20 hours (84 percent of an Earth day) to complete, Kepler 10b has an average orbital distance of only 0.017 AU from its host star and so has a tidally locked, synchronous orbit. Hence, the planet has a perpetually daylight side with a fequilibrium surface temperature of around 2,840 degrees Fahrenheit (1,560 degrees Celsius), which is hotter than molten lava and sufficient to melt iron. Kepler 10b was detected using the transit method from more than eight months of data collected by the spacecraft between May 2009 and early January 2010 and confirmed by radial velocity measurements, and there evidence for another planet (KOI 72.02) in an outer orbit with a period around 45.3 days (Kepler news release; images, animations, and discovery page; and Batalha et al, 2011).

Kepler Mission, NASA

Larger and jumbo illustrations
(more charts and videos).

Multiple planetary transits, including a confirmed
super-Earth (KOI 377.03 or Kepler-9d) with a
1.6-day period and 1.5 times Earth's diameter,
may have been detected for the first time by
the Kepler Mission (more).

On August 25, 2010, NASA announced that the Kepler spacecraft had detected the first multiple transits across a star other than the Sun, towards Constellation Lyra. The Kepler observations indicate that two planets of sub-Saturn size orbit the star designated "Kepler-9" (or KOI-377), where the planet "Kepler-9b" orbits closer to the star with an period of about 19.2 days, while aouter planet "Kepler-9c" has an orbit lasting about 38.9 days. In addition, there are indications of an innermost planetary candidate of super-Earth size (Kepler Object of Interest / KOI 377.03 or "Kepler-9d"), with only 1.64 +0.19/-0.14 times Earth's diameter and a scorching orbit that lasts only 1.6 days, which has been confirmed (Torres et al, 2011; NASA press release, with supporting charts and videos; CfA news release; NASA Science News; and Holman et al, 2010).

Tim Pyle, Kepler Mission, NASA




Larger and jumbo illustrations




The planetary candidate KOI 377.03 (or Kepler-9d),
a super-Earth orbits too close to its host star to be
potentially habitable (more).

Kepler Mission, NASA




Larger illustration.




The Kepler Mission has detected the
possible transits of several hundred
potential super-Earth- and Earth-sized
planets around distant stars (more).