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 confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

orbiting around the star Kepler-78. At the time of its discovery, it was the exoplanet most similar to Earth in terms of

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

, and mean

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

Discovery

Kepler-78b is the only planet to be found orbiting the star KIC 8435766, now known as Kepler-78. The planet was discovered in 2013 by analyzing data from the

Kepler space telescope The Kepler space telescope is a defunct 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

, and mean

. 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 The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experi ...

. 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. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranu ...

, 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 __NOTOC__ 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 Roqu ...

) spectrograph at the

Telescopio Nazionale Galileo The Galileo National Telescope, (; 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 by the "Fundación Galileo Galil ...

in the

Canary Islands The Canary Islands (; ) or Canaries are an archipelago in the Atlantic Ocean and the southernmost Autonomous communities of Spain, Autonomous Community of Spain. They are located in the northwest of Africa, with the closest point to the cont ...

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 A university () is an institution of tertiary education and research which awards academic degrees in several academic disciplines. ''University'' is derived from the Latin phrase , which roughly means "community of teachers and scholars". Univ ...

, used data from the High Resolution Eschelle Spectrometer at the W.M. Keck Observatory in

Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...

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-36b and CoRoT-7b. Lava World Kepler 78b

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 is to the Sun, the planet's surface is estimated to be at a temperature of . This temperature is high enough to have stripped the planet of any stable

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

, 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 rather than an Earth-like planet".

Origin

According to

Harvard–Smithsonian Center for Astrophysics The Center for Astrophysics , Harvard & Smithsonian (CfA), previously known as the Harvard–Smithsonian Center for Astrophysics, is an astrophysics research institute jointly operated by the Harvard College Observatory and Smithsonian Astrophy ...

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 Near-Earth-sized exoplanets Ultra-short period planets