The possibility of an Earth-mass planet in the habitable zone of the Kepler-68 system.

The Kepler Space Telescope has located many multi-planet systems since its inception, which combined with discoveries made by other planet-hunting missions has enabled scientists to begin to construct models of planetary systems orbiting other stars. This is particularly complicated where not all planets are visible to the space telescope, which is only capable of directly detecting fairly large planets. One such system is Kepler-68, where two large planets orbiting close to the star have been directly observed by the telescope as they pass in front of it, and the presence of a third, larger and more distant, planet has been inferred by the actions of its gravity upon the star and two observable planets.

In a paper published on the online arXiv database at Cornell University Library on 9 November 2015 and submitted for publication in the Astrophysical Journal Letters Stephen Kane of the Department of Physics & Astronomy at SanFrancisco State University describes a model of the Kepler-68 system, which calculates the masses and orbits of the planets, as well as plotting the system's Habitable Zone (the zone in which an Earth-mass planet could potentially host liquid water), and the possibility of a small rocky planer orbiting within that zone.

Kepler-68 A (when naming bodies in other stellar systems stars are indicated with an upper case letter while planers are indicated with lower case letters) is a Sun-like star with a mass equivalent to 1.079 times that of the Sun and an effective surface temperature of 5793 K (compared to 5778 K for the Sun). It has a slightly larger radius, 1.243 times that of the Sun, and is somewhat brighter, with a luminosity 1.564 times the Sun's.

The two inner planets of Kepler-68 A, Kepler-68 b and Kepler-68 c, have orbital periods of 5.399 and 9.605 days respectively, indicating that they orbit at 0.061 AU and 0.091 AU (i.e. 6.1 and 9.1% of the distance at which the Earth orbits the Sun), and are calculated to have masses equivalent to 8.3 and 4.8 times that of the Earth.

The third, inferred, planet, Kepler-68 c, is calculated to have a mass equivalent to 0.947 times that of Jupiter, and to orbit Kepler-68 A every 580 days, giving it an average orbital distance of 1.4 AU (1.4 times the distance at which the planer Earth orbits the Sun). However the gravitational influences exerted suggest that its orbit is not circular, rather has an eccentric orbit that takes it from 1.15 AU from the star at its closest to 1.65 AU at its furthest.

Kane calculated two possible ranges for the habitable zone of the Kepler-68 system, a conservative estimate, in which an Earth-like planet would be expected to host liquid water, and an optimistic estimate, within which a small rocky planet could possibly host liquid water. The conservative estimate ranges from 1.19 AU to 2.09 AU from the star, while the optimistic estimate ranges from 0.94 AU to 2.21 AU.

A top-down view of the Kepler-68 system showing the extent of the Habitable Zone and orbits of the planets. The physical scale depicted is 3.36 AU on a side. The conservative Habitable Zone is shown as light-gray and optimistic extension to the Habitable Zone is shown as dark-gray. The inner-most (unlabeled) orbit is that of planet b. Kane (2015).

This means that the distinctly un-Earth-like Kepler-68 c orbits entirely within the conservative habitable-zone of the system, which greatly reduces the possibility of an Earth-like planet aslo being found; the gravitational influence of very large planets makes it hard for smaller planets to occupy nearby orbits without being nudged onto completely different paths, and most likely being wither thrown out of the system altogether or falling into the star. However Kane calculates that a stable zone does exist within the habitable-zone that could host an Earth-like planet, between 1.8 AU and 1.9 AU from the star (outside the orbit of Kepler-68 c). This is in addition to the possibility of an Earth-like moon orbiting the large planet itself (a popular scenario in science fiction movies, but not one all planetary scientists are convinced is possible).

See also

Generating free oxygen in the atmosphere of exoplanets without the presence of life.           In the past two decades over a thousand planets have been found orbiting stars other than our own, many of which appear to be small rocky planets in the habitable zones of their stars (i.e. the zone in which such a...

Kepler-432: a Red Giant Star with at least two giant planets.                                                            Stars form from when vast clouds of gas and dust condense under their own gravity and contract into a single body. As this body contracts it eventually becomes so hot and dense that hydrogen atoms begin to fuse to form helium atoms in its core. This produces massive amounts of energy in the form of heat and light, that push against the gravity of the collapsing...

Determining the Habitable Zone of 70 Virginis.                                                                         70 Viriginis is a G-type Yellow Dwarf Star about 59 light years from Earth in the constellation of Virgo. It is calculated to have a mass 109% of that of the Sun, but radius 194% of the Sun’s, and a lower temperature, 5393K, compared to 5778K for the Sun, from which it is calculated to be somewhat older, approximately 7.77 billion years (compared to about 5.0 for the Sun)...

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Kepler 63b; a giant planet in a polar orbit.

KIC 11554435 (or Kepler Input Catalogue 11554435) is a young, sunlike star 652 light years from the Earth. It is thought to be between 200 and 300 million years old with a mass 98% of the Sun's and an effective surface temperature of ~5576 K, as opposed to 5778 K for the Sun. As a young star it rotates considerably faster than the Sun, roughly once every 5.4 days, while the Sun takes slightly under 24.5 days to complete a rotation, and has a high level of starspot activity (the same as sunspot activity but on a different star).

In a paper published on the online arXiv database at Cornell University Library on 26 August 2013, and accepted for publication in The Astrophysical Journal, a team of scientists led by Roberto Sanchis-Ojeda of the Department of Physics and Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology, describe the discovery of a superjovian planet in orbit about KIC 11554435 by the Kepler Space Telescope. The system is renamed Kepler 63, with the star becoming Kepler 63A and the planet Kepler 63b (naming conventions dictate that stars are given upper case letters and planets lower case letters).

Kepler 63b orbits Kepler 63A every 9.43 days at an estimated distance from the star of 0.08 AU (8% of the distance at which the Earth orbits the Sun), making it a 'Hot Jupiter' type planet, transiting the star (passing in front of it from out point of view) as it does so. It has a highly oblique orbit, at 104° to the plane of the star's rotation. Kepler 63b has a radius 6.1 times that of Jupiter, and while it was not possible to determine its mass, this was given an upper limit of 120 times that of Jupiter.

Sanchis-Ojeda et al. were also able to detect a particularly large and apparently permanent starspot (or cluster of starspots) close the the pole of rotation of Kepler 63A that is turned towards us. This moves around the pole with the star's rotation, so that it is sometimes in line with the orbit of the planet and sometimes not.

Locations of the spot, transit chord, and planet at midtransit, according to the best-fitting model. Sanchis-Ojeda et al. (2013).

See also A superjovian exoplanet directly imaged orbiting the A-class pre main-sequence star HD 95086, The debris disk of 49 Ceti, The planets of NN Serpentis, Water and Hydroxides in the Circumstellar Disk around HD 163296 and The formation of a Keplerian Disk in the L1551 NE Protostellar System.

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NASA's Solar Dynamic Observatory observes the transit of Venus.

A planet is said to transit the Sun when it passes in front of the Sun from our point of view. From Earth it is possible to witness two planets transiting the Sun, Mercury and Venus, since these planets orbit closer to the Sun than the Earth; from Mars it is also possible to see Earth transiting the Sun, etc. Venus, seen from Earth, transits the Sun four times every 243 years, on a cycle of 125.5 years, eight years, 105.5 years, eight years, 125.5 years, and so on. The most recent of these transits occurred on 5-6 June 2012), provoking excitement among both amateur and professional astronomers. This will not happen again till December 2117.

NASA's Solar Dynamic Observatory constantly observes the Sun from a geosynchronous orbit 36 000 above the Pacific Ocean. This placed it in an excellent position to observe the transit of Venus, untroubled by the clouds which bedeviled many Earth-bound observers.

Composite image of Venus transiting the Sun at a wavelength of 171 Å. NASA/Solar Dynamic Observatory.

As well as providing us with dramatic images, this also allows the Solar Dynamic Observatory to perform useful science. The Observatory observes the Sun at a number of different wavelengths. All gasses are opaque at some wavelengths and transparent at others. Therefore by calculating how much light is absorbed by the atmosphere of Venus at different wavelengths, scientists hope to be able to learn more about the composition of that atmosphere.

Film of the 2012 transit of Venus at a wavelength of 171 Å. NASA/Solar Dynamic Observatory.

This space-based view of the transit also tells us something about possible future observations. The next observable transit of Venus will not occur until December 2117, but this is not necessarily the next time we will see this event. Venus (and any other body in the Solar System) is always transiting the Sun from some point of view. It is possible that in the next century humans will come to explore the Solar System far more extensively than we have done to date. While it is unlikely that we will ever send a mission specifically to observe a transit of Venus, it is quite possible that the next humans to observe such a transit will not be on Earth, but rather in space visiting, or on route to, some other body in out Solar System.

See also NASA releases digitally remastered view of Copernicus Crater, New Cassini Images of Titan, Tethys and Methone, Images of Vesta, Martian Dust Devils and NASA releases new Cassini images of Saturn's moons Enceladus, Janus and Dione.

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Looking for HD 97658b.

HD 97658b is a planet with a mass 8.2 times that of the Earth, orbiting a K-type star (HD 97658b) 69 light years from Earth. It was discovered in 2010 by the High Resolution Echelle Spectrometer at the Keck Observatory; its discovery was reported in 2011 by a team of scientists lead by Andrew Howard of the Department of Astronomy and Space Sciences Laboratory at the University of California, Berkeley, and reported in a paper in The Astrophysics Journal. The planet was discovered using the radial-velocity technique: it orbits a star with a mass 85% of that of the Sun every 9.5 days, this causes the star to wobble on its access, enabling astronomers to detect the planet and calculate its mass.

Diagram showing the orbits of HD 97658b (white) and Earth (green) about their respective stars. Planet and star not to scale. allplanets.ru.

Later in 2011, in a paper on the online arXiv database at Cornell University Library, which was also submitted to The Astrophysics Journal, a team lead by Gregory Henry of Center of Excellence in Information Systems at Tennessee State University described observations of HD97685b transiting its star (passing in front of the star when seen from Earth) by the Automated Photometric Telescopes at Fairborn Observatory, from which they calculated that it has a radius of 2.93 times that of the Earth, which translates to a volume of 105 times the Earth's.

In a new paper published on arXiv on 14 April 2012 and submitted for publication in Astrophysical Journal Letters, a team of astronomers lead by Diana Dragomir of the Department of Physics and Astronomy at the University of British Columbia publish the results of a new study of HD 97658b, using the MOST Space Telescope.

An artist's impression of the MOST Space Telescope. University of British Columbia.

The new study was unable to detect HD 97658b transiting the star, despite using a more sensitive instrument. They calculate that the MOST Space Telescope would have been able to detect such transits if the planet had a radius 1.87 times that of the Earth or greater. They therefore conclude that the Fairborn Observatory study was erroneous; either HD 97658b does not transit its star, or it is too small to have been detected by the instruments at Fairborn.

See also Planets discovered orbiting the ancient, second generation star HIP 11952, The atmosphere of WASP-24b, Cooking the planets if CoRoT-7, The origin of the cthonian planets orbiting KIC 05807616 and Exoplanets on Sciency Thoughts YouTube.

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