Kepler-78b (formerly known as KIC 8435766 b) is an
exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
orbiting around the star
Kepler-78
Kepler-78 (formerly known as KIC 8435766) is a 12th magnitude star in the constellation Cygnus (constellation), Cygnus. Initially classified as an eclipsing binary with orbital period 0.710015 day, days, it was later re-classified as the s ...
. At the time of its discovery, it was the exoplanet most similar to Earth in terms of
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
,
radius
In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
, and mean
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
.
Discovery
Kepler-78b is the only planet to be found orbiting the star KIC 8435766, now known as
Kepler-78
Kepler-78 (formerly known as KIC 8435766) is a 12th magnitude star in the constellation Cygnus (constellation), Cygnus. Initially classified as an eclipsing binary with orbital period 0.710015 day, days, it was later re-classified as the s ...
.
The planet was discovered in 2013 by analyzing data from the
Kepler space telescope
The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...
. The planet was detected as it passed across the surface of its host star, as viewed from Earth. It was also found by the effects of occultation as it passed behind the star. Reflected light from the parent star due to orbital phases was also detected. It was not at first designated as a Kepler object of interest, as data analysis failed to identify it due to its short orbital period.
Characteristics
Size, mass, and composition
At the time of its discovery, Kepler-78b was the exoplanet most similar to Earth in terms of
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...
,
radius
In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
, and mean
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
.
The planet is approximately 1.69 times the mass and 1.12 times the radius of Earth.
The acceleration due to gravity on the planet's surface is about , slightly greater than Earth's
surface gravity.
Two independent teams were involved in pioneering work to estimate the planet's mass. Their estimates were made possible because Kepler-78b's gravity causes a "wobble" in the orbit of the host star. While the method has been used to characterize
gas giants
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant ...
, it is difficult to estimate the mass of Earth-sized exoplanets, because their gravity is too weak to produce a visible influence. In this case, the planet's orbit is close enough to its star to produce a detectable effect.
One team, led by Francesco Pepe, used the High Accuracy Radial Velocity Planet Searcher-North (
HARPS-N
HARPS-N, the ''High Accuracy Radial velocity Planet Searcher for the Northern hemisphere'' is a high-precision radial-velocity spectrograph, installed at the Italian Telescopio Nazionale Galileo, a 3.58-metre telescope located at the Roque de los ...
) spectrograph at the
Telescopio Nazionale Galileo
The Galileo National Telescope, ( it, Telescopio Nazionale Galileo; TNG; code: Z19) is a 3.58-meter Italian telescope, located at the Roque de los Muchachos Observatory on the island of La Palma in the Canary Islands, Spain. The TNG is operated b ...
in the
Canary Islands
The Canary Islands (; es, Canarias, ), also known informally as the Canaries, are a Spanish autonomous community and archipelago in the Atlantic Ocean, in Macaronesia. At their closest point to the African mainland, they are west of Morocc ...
to estimate that the planet has a mass 1.86 times that of the Earth and a radius 1.16 times greater. The other, led by Andrew Howard of the
University of Hawaii at Manoa, used data from the High Resolution Eschelle Spectrometer on the
Keck 1 Telescope at the
W.M. Keck Observatory
The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and when comp ...
in
Hawaii
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
to estimate the mass as 1.69 times that of the Earth and the radius as 1.12 times larger. Both estimates put the planet's density at about , equivalent to Earth's density. This measurement is possibly indicative of a rock-iron composition like Earth's.
The iron core could build up to 40% of the planet mass.
Kepler-78b is most similar to larger high-density, hot exoplanets like
Kepler-10b
Kepler-10b is the first confirmed terrestrial planet to have been discovered outside the Solar System by the Kepler Space Telescope. Discovered after several months of data collection during the course of the NASA-directed Kepler Mission, which ...
,
Kepler-36b
Kepler-36b is an exoplanet orbiting the star Kepler-36. This planet has the closest conjunction to Kepler-36c every 97 days. Its density is similar to that of iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and ato ...
and
CoRoT-7b
CoRoT-7b (previously named CoRoT-Exo-7b) is an exoplanet orbiting the star CoRoT-7 in the constellation of Monoceros (constellation), Monoceros, from Earth. It was first detected Methods of detecting extrasolar planets#Transit photometry, photome ...
.
Environment
Kepler-78b orbits around its parent star once every 8.5 hours. It reflects 20% to 60% of the starlight it receives.
Due to its extremely close solar orbit, which is about 40 times closer than
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
is to the Sun, the planet's surface is estimated to be at a temperature of to .
This temperature is high enough to have stripped the planet of any stable
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
, but the liquid and solid portions of the planet should be stable.
According to Francesco Pepe, the planet may be Earth-sized, but "it can be imagined like a
lava planet
A lava planet is a type of terrestrial planet, with a surface mostly or entirely covered by molten lava. Situations where such planets could exist include a young terrestrial planet just after its formation, a planet that has recently suffered a ...
rather than an Earth-like planet".
Origin
According to
Harvard–Smithsonian Center for Astrophysics
The Center for Astrophysics , Harvard & Smithsonian (CfA), alternatively called the Harvard–Smithsonian Center for Astrophysics, is an astrophysics research institute jointly operated by the Harvard College Observatory and Smithsonian Astrop ...
astronomer Dimitar Sasselov, "this lava world is an abomination. There’s no physical way a small world, only 12 percent larger than Earth, could have evolved in that location and there’s no known mechanism that could have transported it there. But one thing that is certain, it can’t stay roasting in that hellish orbit for long; it’s destined to get swallowed by its star very soon". It is estimated that the planet will be swallowed by its parent star in about three billion years.
References
External links
Notes on the planet KIC 8435766 b
{{2013 in space
Exoplanets discovered in 2013
Exoplanets discovered by the Kepler space telescope
Transiting exoplanets
Cygnus (constellation)
Terrestrial planets