Super-Earths
Super-Earths |
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Kepler Mission,
NASA
Larger illustration.
Super-Earths can have
a surface layer of
water or rock
(more).
Breaking News
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, 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). Potential super-Earths -- with estimated diameters (or radii) between 1.5 and 2.2 that of Earth's -- that were found and released by the Kepler Mission as part of its 306 stars with planetary candidates are listed in the two lower tables below by designation number as a "Kepler Object of Interest" (KOI).
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 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 announced the finding of a similarly sized planet around a main sequence star, with the discovery of Gliese (Gl) 876 d.
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).
Super-Earths within 100 light-years
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)
Day
Temp.
(F)
Day
Temp.
(C)
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
20.4 M2.5 V Gl 581 c 5.4-10.4 >1 0.07 0.11-0.28 12.9 0.035 ~0.2 hot hot
20.4 M2.5 V Gl 581 d 7.1-13.8 =>1.5 0.22 0.11-0.28 66.8 0.183 ~0.4 27-104+ (3)-40+
20.4 M2.5 V Gl 581 e 1.9-3.1 >1 0.03 0.11-0.28 3.1 0.085 ~0 hot hot
22.7 M1.5-
2.5 VGl 667 c =>5.7 >1 ~0.05 ~0.1-0.3+ ~7 ~0.02 ? hot hot
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
40.9 G8-K0 V 55 Cancri e 10.8-13.5 >1 0.038 0.7-1.4 2.8 0.008 0.07 2,700 1,500
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 2.55-2.81 0.014 ~0.1-0.2 1.6 0.0044 <0.27 hot hot
54.0 K0V HD 7924 b =>9.3 >1 0.057 0.5-1.1 5.4 0.015 ~0.2 hot hot
76.4 G0 V HD 1461 b =>7.6 >1 0.063 0.9-1.9 5.8 0.016 0.04 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
... ... ... ... ... ... ... ... ... ... ... ...
Notable Super-Earths beyond 100 light-years
Distance
from Sol
(ly)
Primary
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)
Day
Temp.
(F)
Day
Temp.
(C)
~142 K3 V HD 215497 b =>5.4 >1 >0.1 0.97-1.29 3.9 0.011 ? hot hot
~490 G9-K0 V CoRoT-7 b ~4.8 1.7 0.017 <1 0.85 0.0023 ~0 2,200-3,700 1,500-2,300
~980 pulsar PSR B1257+12 c 4.3 >1? 0.38 none 66 0.18 0.19 frigid frigid
~980 pulsar PSR B1257+12 d 3.9 >1? 0.46 none 98 0.27 0.25 frigid frigid
~1,000 brown
dwarf?MOA-2007-
BLG-192L b~3.3 >1 ~0.6? <0.01 ? <1 ? frigid frigid
~22,000 M4? V OGLE-2005-
BLG-390L b~5.5 >1 ~2.6 ~0.1-0.2 ~3,300 ~8.9 ? (370) (220)
? G? KOI 191.02 5-18 2.0 <0.1 ? 2.4 0.007 ? hot hot
? G? KOI 877.01 6-40 2.6 <0.1 ? 6.0 0.016 ? hot hot
? G? KOI 877.02 5-25 2.3 <0.1 ? 12.0 0.033 ? hot hot
... ... KOI ? ... ... ... ... ... ... ... ... ...
... ... ... ... ... ... ... ... ... ... ... ...
Luis Calçada, ESO
Large and
jumbo illustrations.
CoRoT-7 b, the first confirmed
rocky planet detected outside
the Solar system, may be the
remnant core of a Saturn-mass
gas giant that had its atmosphere
and lower gas metal layers
burned off
(more)
Other Possible Super-Earths Found by Kepler
Distance
from Sol
(ly)
Primary
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)
Day
Temp.
(F)
Day
Temp.
(C)
? ? KOI 217.01 ? 2.0 ? ? 5.6 0.015 ? hot hot
? ? KOI 237.01 ? 2.2 ? ? 8.5 0.023 ? hot? hot?
? ? KOI 241.01 ? 2.1 ? ? 13.8 0.038 ? ? ?
? ? KOI 422.01 ? 2.1 ? ? 12.8 0.035 ? ? ?
? ? KOI 438.01 ? 2.1 ? ? 5.9 0.016 ? hot hot
? ? KOI 457.01 ? 2.1 ? ? 4.9 0.013 ? hot hot
? ? KOI 471.01 ? 1.9 ? ? 21.3 0.058 ? ? ?
? ? KOI 484.01 ? 2.2 ? ? 17.2 0.047 ? ? ?
? ? KOI 487.01 ? 1.9 ? ? 7.7 0.021 ? hot hot
? ? KOI 488.01 ? 2.0 ? ? 9.4 0.026 ? hot hot
? ? KOI 491.01 ? 1.7 ? ? 4.7 0.013 ? hot hot
? ? KOI 494.01 ? 1.9 ? ? 25.7 0.070 ? ? ?
? ? KOI 499.01 ? 1.9 ? ? 9.7 0.026 ? hot hot
? ? KOI 502.01 ? 2.0 ? ? 5.9 0.016 ? hot hot
? ? KOI 504.01 ? 1.9 ? ? 40.6 0.111 ? ? ?
? ? KOI 514.01 ? 1.9 ? ? 11.8 0.032 ? hot? hot?
? ? KOI 522.01 ? 2.1 ? ? 12.8 0.035 ? ? ?
? ? KOI 524.01 ? 2.2 ? ? 4.6 0.013 ? hot hot
? ? KOI 537.01 ? 2.0 ? ? 2.8 0.008 ? hot hot
? ? KOI 539.01 ? 1.7 ? ? 29.1 0.080 ? ? ?
? ? KOI 541.01 ? 1.7 ? ? 13.6 0.080 ? ? ?
? ? KOI 543.01 ? 1.9 ? ? 4.3 0.012 ? hot? hot?
? ? KOI 544.01 ? 1.7 ? ? 3.7 0.010 ? hot? hot?
? ? KOI 551.01 ? 1.9 ? ? 11.6 0.032 ? hot? hot?
? ? KOI 553.01 ? 2.2 ? ? 2.4 0.007 ? hot hot
? ? KOI 559.01 ? 1.9 ? ? 4.3 0.012 ? hot hot
? ? KOI 560.01 ? 1.8 ? ? 23.7 0.065 ? ? ?
? ? KOI 563.01 ? 2.0 ? ? 15.3 0.042 ? ? ?
? ? KOI 565.01 ? 1.6 ? ? 2.3 0.007 ? hot hot
? ? KOI 580.01 ? 2.1 ? ? 6.5 0.018 ? hot hot
? ? KOI 582.01 ? 2.2 ? ? 5.9 0.016 ? hot hot
? ? KOI 583.01 ? 1.9 ? ? 2.4 0.007 ? hot hot
? ? KOI 585.01 ? 2.0 ? ? 3.7 0.010 ? hot hot
? ? KOI 586.01 ? 1.8 ? ? 15.8 0.043 ? ? ?
? ? KOI 590.01 ? 1.8 ? ? 11.4 0.031 ? hot hot
? ? KOI 593.01 ? 2.1 ? ? 10.0 0.027 ? hot? hot?
? ? KOI 597.01 ? 2.2 ? ? 17.3 0.047 ? ? ?
? ? KOI 598.01 ? 2.0 ? ? 8.3 0.023 ? hot? hot?
? ? KOI 599.01 ? 2.2 ? ? 6.4 0.018 ? hot hot
? ? KOI 600.01 ? 2.0 ? ? 3.6 0.010 ? hot hot
? ? KOI 605.01 ? 1.8 ? ? 2.6 0.007 ? hot hot
? ? KOI 610.01 ? 2.1 ? ? 14.3 0.039 ? ? ?
? ? KOI 740.01 ? 1.9 ? ? 17.7 0.048 ? ? ?
? ? KOI 750.01 ? 2.1 ? ? 21.7 0.059 ? ? ?
? ? KOI 765.01 ? 2.2 ? ? 8.4 0.023 ? hot? hot?
? ? KOI 778.01 ? 2.0 ? ? 2.2 0.006 ? hot hot
? ? KOI 786.01 ? 1.9 ? ? 3.7 0.010 ? hot hot
? ? KOI 789.01 ? 1.7 ? ? 14.1 0.039 ? ? ?
? ? KOI 790.01 ? 2.0 ? ? 8.5 0.023 ? hot? hot?
? ? KOI 825.01 ? 2.1 ? ? 8.1 0.022 ? hot? hot?
? ? KOI 835.01 ? 1.9 ? ? 11.8 0.032 ? ? ?
? ? KOI 837.01 ? 1.8 ? ? 8.0 0.022 ? hot? hot?
? ? KOI 852.01 ? 2.2 ? ? 3.8 0.010 ? hot hot
? ? KOI 857.01 ? 2.1 ? ? 5.7 0.016 ? hot hot
? ? KOI 873.01 ? 1.6 ? ? 4.3 0.012 ? hot hot
? ? KOI 874.01 ? 1.9 ? ? 4.6 0.013 ? hot hot
? ? KOI 911.01 ? 2.0 ? ? 4.1 0.011 ? hot hot
? ? KOI 920.01 ? 1.7 ? ? 21.8 0.060 ? ? ?
? ? KOI 937.01 ? 2.2 ? ? 20.8 0.057 ? ? ?
? ? KOI 948.01 ? 2.1 ? ? 24.6 0.067 ? ? ?
... ... ... ... ... ... ... ... ... ... ... ...
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