Kepler Object of Interest
Encyclopedia
A Kepler Object of Interest (KOI) is a star observed by the Kepler spacecraft which is suspected of hosting one or more transiting planets
. KOIs come from a master list of 150,000 stars which itself is generated from the Kepler Input Catalog
(KIC). A KOI shows a periodic dimming, indicative of an unseen planet passing between the star and Earth, eclipsing
part of the star. However, such an observed dimming is not a guarantee of a transiting planet, as other astronomical objects - such as an eclipsing binary in the background - can mimic a transit signal. For this reason, the majority of KOIs are not as yet confirmed transiting planet systems.
On February 1, 2011, a second release of observations made during the same time frame contained improved date reduction and listed 1235 transit signals around 997 stars.
, surface gravity
and mass
. These quantities are derived from photometric
observations taken prior to Kepler's launch at the 1.2 m reflector at Fred Lawrence Whipple Observatory
. For KOIs, there is, additionally, data on each transit signal: the depth of the signal, the duration of the signal and the periodicity
of the signal (although some signals lack this last piece of information). Assuming the signal is due to a planet, these data can be used to obtain the size of the planet relative to its host star, the planet's distance from the host star relative to the host star's size (assuming zero eccentricity
), and the orbital period of the planet. Combined with the estimated properties of the star described previously, estimates on the absolute size of the planet, its distance from the host star and its equilibrium temperature can be made.
. In cases such as these, there is more surface area producing light than is assumed, so a given transit signal is larger than assumed. Since roughly 34% of stellar systems are binaries, up to 34% of KOI signals could be from planets within binary systems and, consequently, be larger than estimated (assuming planets are as likely to form in binary systems as they are in single star systems). However, additional observations can rule out these possibilities and are essential to confirming the nature of any given planet candidate.
measurements of the planet acting on the KOI. However, for many KOIs this is not feasible. In these cases, speckle imaging
or adaptive optics
imaging using ground based telescopes can be used to greatly reduce the likelihood of background eclipsing binaries. Such follow-up observations are estimated to reduce the chance of such background objects to less than 0.01%. Additionally, spectra
of the KOIs can be taken to see if the star is part of a binary system.
has a second signal KOI 72.02 that is as yet not confirmed to be a planet.
, KOI 2
, and KOI 3
.
, KOI 18
, KOI 17
, KOI 97
, KOI 10
, KOI 377
, KOI 72
, and KOI 157
. Of these, KOI 377
and KOI 157
have multiple planets (3 and 6, respectively) confirmed to be orbiting them.
.
Six transit signals released in the February 1, 2011 data are indicative of planets that are both "Earth-like" (less than 2 Earth radii in size) and located within the habitable zone
of the host star. They are: KOI 1026.01, KOI 854.01, KOI 701.03, KOI 268.01, KOI 326.01, and KOI 70.03 .A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported.
A September 2011 study by Muirhead et al reports that a re-calibration of estimated radii and effective temperatures of several dwarf stars in the Kepler sample yields six new Earth-sized candidates within the habitable zones of their stars: KOI 463.01, KOI 1422.02, KOI 947.01, KOI 812.03, KOI 448.02, KOI 1361.01.http://arxiv.org/abs/1109.1819
. These objects include KOI 74
, KOI 81
and KOI 959.
KOI 54 is believed to be a binary system containing two A-stars in highly eccentric orbits with a semi-major axis
of 0.4 AU
. During periastron, tidal distortions
cause a periodic brightening of the system. In addition, these tidal forces induce resonant pulsations in one (or both) of the stars, making it only the 4th known stellar system to exhibit such behavior.
KOI 126 is a triple star system comprising two low mass (0.24 and 0.21 solar masses) stars orbiting each other with a period of 1.8 days and a semi-major axis of 0.02 AU. Together, they orbit a 1.3 solar mass star with a period of 34 days and a semi-major axis of 0.25 AU. All three stars eclipse one another which allows for precise measurements of their masses and radii. This makes the low mass stars 2 of only 4 known fully convective stars to have accurate determinations of their parameters (i.e. to better than several percent). The other 2 stars constitute the eclipsing binary system CM Draconis
.
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
. KOIs come from a master list of 150,000 stars which itself is generated from the Kepler Input Catalog
Kepler Input Catalog
The Kepler Input Catalog is a publicly searchable database of roughly 13.2 million targets used for the Kepler Spectral Classification Program and Kepler....
(KIC). A KOI shows a periodic dimming, indicative of an unseen planet passing between the star and Earth, eclipsing
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
part of the star. However, such an observed dimming is not a guarantee of a transiting planet, as other astronomical objects - such as an eclipsing binary in the background - can mimic a transit signal. For this reason, the majority of KOIs are not as yet confirmed transiting planet systems.
History
The first public release of a list of KOIs was on 15 June 2010 and contained 306 stars suspected of hosting exoplanets, based on observations taken between 2 May 2009 and 16 September 2009. It was also announced that an additional 400 KOIs had been discovered, but would not be immediately released to the public. This was done in order for follow-up observations to be performed by Kepler team members..On February 1, 2011, a second release of observations made during the same time frame contained improved date reduction and listed 1235 transit signals around 997 stars.
Naming convention
Stars observed by Kepler that are considered candidates for transit events are given the designation "KOI" followed by an integer number. For each set of periodic transit events associated with a particular KOI, a two-digit decimal is added to the KOI number for that star. For example, the first transit event candidate identified around the star KOI 718 is designated KOI 718.01, while the second candidate is KOI 718.02 and the third is KOI 718.03. Once a transit candidate is verified to be a planet (see below), the star is designated "Kepler" followed by a hyphen and an integer number. The associated planet(s) have the same designation, followed by a letter in the order each was discovered.Kepler data on KOIs
For all 150,000 stars being watched for transits by Kepler, there are estimates of each star's surface temperature, radiusRadius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
, surface gravity
Surface gravity
The surface gravity, g, of an astronomical or other object is the gravitational acceleration experienced at its surface. The surface gravity may be thought of as the acceleration due to gravity experienced by a hypothetical test particle which is very close to the object's surface and which, in...
and mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
. These quantities are derived from photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
observations taken prior to Kepler's launch at the 1.2 m reflector at Fred Lawrence Whipple Observatory
Fred Lawrence Whipple Observatory
The Fred Lawrence Whipple Observatory is an astronomical observatory owned and operated by the Smithsonian Astrophysical Observatory and is their largest field installation outside of their main site in Cambridge, MA...
. For KOIs, there is, additionally, data on each transit signal: the depth of the signal, the duration of the signal and the periodicity
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the signal (although some signals lack this last piece of information). Assuming the signal is due to a planet, these data can be used to obtain the size of the planet relative to its host star, the planet's distance from the host star relative to the host star's size (assuming zero eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
), and the orbital period of the planet. Combined with the estimated properties of the star described previously, estimates on the absolute size of the planet, its distance from the host star and its equilibrium temperature can be made.
False positives
While it has been estimated that 90% of the KOI transit candidates are true planets, it is expected that some of the KOIs will be false positives, i.e., not actual transiting planets. The majority of these false positives are anticipated to be eclipsing binaries which, while spatially much more distant and thus dimmer than the foreground KOI, are too close to the KOI on the sky for the Kepler telescope to differentiate. On the other hand, statistical fluctuations in the data are expected to contribute less than one false positive event in the entire set of 150,000 stars being observed by Kepler.Misidentification
In addition to false positives, a transit signal can be due to a planet that is substantially larger than what is estimated by Kepler. This occurs when there are sources of light other than simply the star being transited, such as in a binary systemBinary system (astronomy)
A binary system is an astronomical term referring to two objects in space which are so close that their gravitational interaction causes them to orbit about a common center of mass. Some definitions A binary system is an astronomical term referring to two objects in space (usually stars, but also...
. In cases such as these, there is more surface area producing light than is assumed, so a given transit signal is larger than assumed. Since roughly 34% of stellar systems are binaries, up to 34% of KOI signals could be from planets within binary systems and, consequently, be larger than estimated (assuming planets are as likely to form in binary systems as they are in single star systems). However, additional observations can rule out these possibilities and are essential to confirming the nature of any given planet candidate.
Verifying candidates
Additional observations are necessary in order to confirm that a KOI actually has the planet that has been predicted, instead of being a false positive or misidentification. The most well-established confirmation method is to obtain radial velocityRadial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
measurements of the planet acting on the KOI. However, for many KOIs this is not feasible. In these cases, speckle imaging
Speckle imaging
Speckle imaging describes a range of high-resolution astronomical imaging techniques based either on the shift-and-add method or on speckle interferometry methods...
or adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
imaging using ground based telescopes can be used to greatly reduce the likelihood of background eclipsing binaries. Such follow-up observations are estimated to reduce the chance of such background objects to less than 0.01%. Additionally, spectra
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
of the KOIs can be taken to see if the star is part of a binary system.
KOIs with confirmed planets
As of February 8, 2011, 12 KOI stars have a total of 19 confirmed planets orbiting them. In addition, KOI 72Kepler-10
Kepler-10, formerly known as KOI-72, is a sunlike star in constellation Draco that lies 173 parsecs away from the Earth. Kepler-10 was targeted by NASA's Kepler spacecraft, as it was seen the first star identified by the Kepler mission as a possible host to a small, transiting exoplanet...
has a second signal KOI 72.02 that is as yet not confirmed to be a planet.
Previously detected planets
Three stars within the Kepler Spacecraft's field of view have planets which were previously known and which were re-observed by Kepler. These stars are: KOI 1GSC 03549-02811
GSC 03549-02811 A is a yellow main-sequence star similar to our Sun. This star is located approximately 718 light-years away in the constellation of Draco. The apparent magnitude of this star is 11.41, which means it is not visible to the naked eye but can be seen with a medium-sized amateur...
, KOI 2
HAT-P-7
HAT-P-7 is a yellow white dwarf star located about 1044 light-years away in the constellation Cygnus. The apparent magnitude of this star is 10.5, which means it is not visible to the naked eye but can be seen with a small telescope on a clear dark night.-Planetary system:This star has one known...
, and KOI 3
HAT-P-11
HAT-P-11, also designated GSC 03561-02092, is an orange dwarf metal rich star about 123 light-years away in the constellation Cygnus. This star is notable for its relatively large rate of proper motion. The magnitude of this star is about 9, which means it is not visible to the naked eye but can be...
.
Planets confirmed by the Kepler team
Eight stars were first observed by Kepler to have signals indicative of transiting planets and have since had their nature confirmed. These stars are: KOI 7Kepler-4
Kepler-4 is a sunlike star located about 1631 light-years away in the constellation Draco. It is in the field of view of the Kepler Mission, a NASA operation purposed with finding Earth-like planets. Kepler-4b, a Neptune-sized planet that orbits extremely close to its star, was discovered in its...
, KOI 18
Kepler-5
Kepler-5 is a star located in the constellation Cygnus in the field of view of the Kepler Mission, a NASA project aimed at detecting planets in transit of, or passing in front of, their host stars as seen from Earth. One closely orbiting, Jupiter-like planet, named Kepler-5b, has been detected...
, KOI 17
Kepler-6
Kepler-6 is a yellow giant situated in the constellation Cygnus. The star lies within the field of view of the Kepler Mission, which discovered it as part of a NASA-led mission to discover Earth-like planets...
, KOI 97
Kepler-7
Kepler-7 is a star located in the constellation Lyra in the field of view of the Kepler Mission, a NASA operation in search of Earth-like planets. It is home to the fourth of the first five planets that Kepler discovered; this planet, a Jupiter-size gas giant named Kepler-7b, is as light as styrofoam...
, KOI 10
Kepler-8
| class=F5VKepler-8 is a star located in the constellation Lyra in the field of view of the Kepler Mission, a NASA-led operation tasked with discovering terrestrial planets. The star, which is slightly hotter, larger, and more massive than the Sun, has one gas giant in its orbit, Kepler-8b. This...
, KOI 377
Kepler-9
Kepler-9 is a sunlike star in the constellation Lyra. It is situated in the Kepler Mission's field of view. Kepler-9 has three confirmed extrasolar planets, all discovered by Kepler using the transit method. The discovery of the first two planets was announced on August 26, 2010, and of the third...
, KOI 72
Kepler-10
Kepler-10, formerly known as KOI-72, is a sunlike star in constellation Draco that lies 173 parsecs away from the Earth. Kepler-10 was targeted by NASA's Kepler spacecraft, as it was seen the first star identified by the Kepler mission as a possible host to a small, transiting exoplanet...
, and KOI 157
Kepler-11
Kepler-11 is a sun-like star slightly larger than the Sun in the constellation Cygnus, located some 2,000 light years from Earth. It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission uses to detect planets that may be transiting their stars...
. Of these, KOI 377
Kepler-9
Kepler-9 is a sunlike star in the constellation Lyra. It is situated in the Kepler Mission's field of view. Kepler-9 has three confirmed extrasolar planets, all discovered by Kepler using the transit method. The discovery of the first two planets was announced on August 26, 2010, and of the third...
and KOI 157
Kepler-11
Kepler-11 is a sun-like star slightly larger than the Sun in the constellation Cygnus, located some 2,000 light years from Earth. It is located within the field of vision of the Kepler spacecraft, the satellite that NASA's Kepler Mission uses to detect planets that may be transiting their stars...
have multiple planets (3 and 6, respectively) confirmed to be orbiting them.
Planets confirmed by other collaborations
From the Kepler data released to the public, one system has been confirmed to have a planet, KOI 428bKOI-428
KOI-428 is an F-type star with one known planet, KOI-428b, in orbit. The star is approximately 1.5 times more massive than the Sun, and is over two times its size; it was, at upon its discovery, the largest and the most evolved star yet discovered with a transiting planet in its orbit...
.
KOIs with unconfirmed planets
KOI 70 has transit signals indicating the existence of at least four planets. If confirmed, KOI 70.04 would be the smallest extrasolar planet discovered around a main-sequence star (at 0.6 Earth radii) to date, and the second smallest known extrasolar planet after PSR 1257 12 b. The likelihood of KOI 70.04 being of the nature deduced by Kepler (and not a false positive or misidentification) has been estimated at >80%.Six transit signals released in the February 1, 2011 data are indicative of planets that are both "Earth-like" (less than 2 Earth radii in size) and located within the habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...
of the host star. They are: KOI 1026.01, KOI 854.01, KOI 701.03, KOI 268.01, KOI 326.01, and KOI 70.03 .A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported.
A September 2011 study by Muirhead et al reports that a re-calibration of estimated radii and effective temperatures of several dwarf stars in the Kepler sample yields six new Earth-sized candidates within the habitable zones of their stars: KOI 463.01, KOI 1422.02, KOI 947.01, KOI 812.03, KOI 448.02, KOI 1361.01.http://arxiv.org/abs/1109.1819
Non-planet discoveries
Several KOIs contain transiting objects which are hotter than the stars they transit, indicating that the smaller objects are white dwarfs formed through mass transferMass transfer
Mass transfer is the net movement of mass from one location, usually meaning a stream, phase, fraction or component, to another. Mass transfer occurs in many processes, such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. Mass transfer is used...
. These objects include KOI 74
KOI-74
KOI-74 is an eclipsing binary star in the constellation of Cygnus. The primary star is an A-type main sequence star with a temperature of . It lies in the field of view of the Kepler Mission and was determined to have a companion object in orbit around it which is smaller and hotter than the main...
, KOI 81
KOI-81
KOI-81 is an eclipsing binary star in the constellation of Cygnus. The primary star is a late B-type or early A-type main sequence star with a temperature of . It lies in the field of view of the Kepler Mission and was determined to have an object in orbit around it which is smaller and hotter than...
and KOI 959.
KOI 54 is believed to be a binary system containing two A-stars in highly eccentric orbits with a semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
of 0.4 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
. During periastron, tidal distortions
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...
cause a periodic brightening of the system. In addition, these tidal forces induce resonant pulsations in one (or both) of the stars, making it only the 4th known stellar system to exhibit such behavior.
KOI 126 is a triple star system comprising two low mass (0.24 and 0.21 solar masses) stars orbiting each other with a period of 1.8 days and a semi-major axis of 0.02 AU. Together, they orbit a 1.3 solar mass star with a period of 34 days and a semi-major axis of 0.25 AU. All three stars eclipse one another which allows for precise measurements of their masses and radii. This makes the low mass stars 2 of only 4 known fully convective stars to have accurate determinations of their parameters (i.e. to better than several percent). The other 2 stars constitute the eclipsing binary system CM Draconis
CM Draconis
CM Draconis is an eclipsing binary system approximately 47 light-years away in the constellation of Draco . The system consists of two nearly identical red dwarf stars located in the constellation Draco. The two stars orbit each other with a period of 1.27 days with a separation of 2.7 million...
.