First Discoveries: Gamma Cephei A b. Astronomers using NASA's Spitzer Space Telescope have detected what they believe is an alien world just two-thirds the size of Earth one of the smallest on record. VLT image of exoplanet HD b. An artist's illustration of Keplerb or PH1, a planet discovered by volunteers from the Planet Hunters citizen science project.
PH1, shown in the foreground, is the first reported case of a plane Keplerb: Four Star Planet. A truly doomed planet, WASPb is being cannibalized by its star. It's a Star-Eat-Planet World. This illustration shows the unusual orbit of planet Keplerb around a close pair of orange and red dwarf stars. The planet's day orbit is tilted 2.
Wobbly Planet Orbital Schematic Illustration. In this artist's impression, a disk of dusty material leftover from star formation girds two young stars like a hula hoop. As the two stars whirl around each other, they periodically peek out from The Kepler spacecraft, responsible for thousands of new potential disoveries. Kepler Spacecraft. A unique feature of the seventh Kepler candidate catalog is that it is the first to fully automate the assessment of transit-like signals. The total height of each bar shows the total number of Kep Assessment of Kepler Dataset.
While thousands of exoplanet candidates have been discovered, it generally takes at least two independent observations to confirm that these planets actually exist. To be scientifically rigorous, the planet should be observed by two separate teams using two different observatories. The original researchers that found Gliese g in , however, defended their methods in and cast doubts on some of the other teams' work. At least one well-known exoplanet database doesn't list it any more. Using 11 years of observational data from the W.
Keck Observatory in Hawaii , the teams announced two planets around the star Gliese Gliese f and Gliese g. Results were published in the Astrophysical Journal and also made available in pre-print version on Arxiv.
The planet was believed to be within the habitable zone of its parent star, which is a red dwarf. This type of star is cooler than our own sun, which means planets need to huddle close to receive enough warmth for water to flow on their surface.
While astronomers generally define habitability by whether the planet can support liquid water, it is acknowledged that there are many factors that can influence it.
This includes the planet's atmosphere and how variable its parent star is in terms of emitting energy. In a press release announcing the discovery , the researchers acknowledged Gliese "has a somewhat checkered history of habitable-planet claims". Two planets found in the system before, Gliese c and Gliese d, were later believed to be at the edge of the habitable zone.
In future years, Gliese d's existence was also called into question. A planetary object in space without an internal heat source will heat up or cool off until its surface reaches a thermal " equilibrium temperature ," where it emits exactly as much thermal energy as it receives from its host star s. A relatively massive planet like Venus, Earth, or larger super-Earth with a dense atmosphere and sufficient amounts of gases that block re-radiation of infrared heat back into space, however, will warm further from a greenhouse effect.
With such considerations in mind, some astronomers have calculated the habitable zone around Gliese for representative super-Earths to be as wide as between around 0. As of September 29, , Gliese was believed by some astronomers to host at least six planets between the size of Earth and Neptune , with one planetary candidate "g" orbiting the star wholely within its habitable zone whose existence has not been confirmed.
Fourth to be discovered, innermost planetary candidate "e" with two to three times Earth's mass was first detected in more. Planet "e" - On April 21, , a team of astronomers at the European Week of Astronomy and Space Science conference announced the discovery of planetary candidate "e" with a minimum mass initially estimated to be around 1. The fourth object to be discovered around Gliese , planet e is now thought to have at least 1.
It revolves around Gliese -- inside of planet b's orbit -- at an average distance of 0. It is not yet known whether planetary candidate "b" is rocky or gaseous -- like Neptune. Planet "b" - On November 30, , a team of French and Swiss astronomers announced the discovery of a Neptune -class planet in a very close "torch" orbit around this star using the HARPS spectrograph on the European Southern Observatory 's 3.
Initially estimated with at least With a semi-major axis of 0. On Planet "c," water is unlikely to exist on its day-side surface except as lakes or seas on its colder dark side opposite the star under a likely dense atmosphere more.
The more Earth-like planet "c" has at least 5. It moves around Gliese -- outside of planet b's orbit -- at an average distance of 0. Based on its minimum mass and a range of predicted radii, the surface gravity of planet c may lie between 1. Planet c was initially calculated to orbit Gliese near the hot, inner edge of the red dwarf's relative narrow and very close habitable zone. Subsequent models indicated, however, that such a massive planet is likely have a dense atmosphere with a gas composition that should undergo a Venus-like runaway greenhouse effect, which should boil off all surface water Vogt et al, ; Selsis et al, ; and von Bloh et al, , although others have suggested that surface lakes or seas could be possible on the colder, perpetually dark side of planet c see an illustration of such water features on planet c by Lynette Cook.
Illustrations of another artist's vision of day- and night-side surfaces of planet c due to tidally locked, synchronous rotation around host star Gliese are available from Karen Wehrstein and Astronomy Picture of the Day. Planetary candidate "d" orbits just within the outer edge of Gliese 's habitable zone and so may have liquid surface water in a deep global ocean more. Planet "d? Planet "d" is now estimated to have between 5.
It revolves around Gliese -- outside of planet b's orbit -- at an average distance of 0. Although it was once thought possible that this detection was actually caused by a large star spot, its discoverers now believe that the host star is too inactive for this possibility to be likely Vogt et al, ; Vogt et al, ; Mayor et al, ; and Udry et al, Subsequent calculations suggest that a greenhouse effect under the right atmospheric conditions may make it possible for liquid water to persist on its surface despite its tidally locked, synchronous rotation around Gliese von Bloh et al, Orbiting near the outer edge of Gliese 's habitable zone at an orbital distance of just under 0.
As planet d is calculated to have between 5. Models show that when atmospheric pressure rises to at least five bar 5 times Earth's atmospheric pressure at sea level , the resulting greenhouse effect in a high-CO 2 atmosphere can cause temperatures to rise significantly above freezing and thaw surface ice Olaf Stampf, Der Spiegel , October 8, ; Kaltenegger et al, ; von Paris et al, ; Wordsworth et al, ; Vogt et al, ; Selsis et al, ; and von Bloh et al, Model calculations suggest that atmospheric pressure on the planet may reach up to seven or eight bars, a level found on Earth at the bottom of lakes.
If the atmosphere has more than 10 bars of CO 2 and and other gases such as molecular nitrogen, or N 2 , however, three-dimensional climate simulations suggest that the planet's global mean temperatures could rise above freezing for land as well as ocean-covered surfaces Wordsworth et al, ; and Jon Voisey, Universe Today, May 6, Some astronomers also believe that planet d is too massive to be made only of rock; more likely, the planet is ice-rich like Neptune or Uranus which agglomerated large ice layers of water, ammonia, and methane before migratng closer to Gliese , and so its abundant ices could have been warmed enough to melt into a deep global ocean ESO press release.
If planet d does have rocky surfaces, its surface gravity in those areas would be around 1. On July 3, , a team of astronomers submitted a preprint which argued that the stellar activity of Gliese has led to the false detection of planets "d" and "g" through radial-velocity variations. Planetary candidate "g" is believed to be a rocky super-Earth that orbits Gliese within the red dwarf star's habitable zone more. Planet "g" - On September 29, , a team of astronomers including Steven S.
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