Kepler-78 — host star of the first confirmed rocky Earth-sized exoplanet

Kepler-78 is a star in the constellation Cygnus, sitting 407.26 light-years from Earth — a distance so large that no probe humanity has yet launched would cross it within any foreseeable planning horizon. Its radius measures 0.736965 solar radii, making it somewhat smaller than the Sun, and it harbors a planet, Kepler-78b, whose existence current theories of planet formation cannot explain. That planet, confirmed in 2013, was the first Earth-sized world outside the Solar System shown to share Earth’s rocky, iron-rich composition — and the mechanism by which it arrived in its current orbit remains an open question in planetary science.

Key facts

Mass: 816.26 Jupiter masses
Body type: star; near-infrared source
Radius: 0.736965 solar radius
Distance: 407.26 light-years

The Star

Kepler-78 is classified as a star and a near-infrared source. Its radius of 0.736965 solar radii places it noticeably below the Sun in size — small enough that the difference is physically meaningful when considering what the young, inflated star’s interior once occupied in space. Its mass is 816.26 Jupiter masses.

The star lies at a distance of 407.26 light-years. A light-year is a unit of distance, not a unit of time, and at 407.26 of them Kepler-78 is far enough that it is observable only through its light and the periodic signals that its planet imprints on that light.

Discovery and Catalog History

Kepler-78 first appeared in the scientific literature on October 12, 2011, in the second data release of the Kepler Eclipsing Binary catalog, where it was listed under Kepler Input Catalog number 8435766. At that stage it was misclassified as a detached eclipsing binary star with a period of 0.71 days. That period turned out to be exactly twice the true orbital period of the planet Kepler-78b, which is 0.355 days — a factor-of-two error that caused automated analysis to reach the wrong conclusion and delayed recognition of the system’s real nature.

The planetary signal was eventually recovered by a team that applied a Fourier Transform method rather than the Box-fitting Least Squares algorithm used by the main Kepler pipeline. The standard pipeline’s algorithm was not sensitive enough to catch the ultra-short-period signal, which is why the discovery came later than the raw photometric data would otherwise have permitted.

Kepler-78b: An Ultra-Short-Period Planet

Kepler-78b belongs to the class of ultra-short-period (USP) planets identified by the Kepler spacecraft — worlds with orbital periods shorter than one day and radii typically below 2 Earth radii. Kepler-78b was the first USP planet to have its mass directly measured. Its orbital period of 0.355 days means the planet completes a full circuit of its star in about 8.5 hours, a pace that places it in an entirely different regime from anything in the Solar System.

The orbit itself is extraordinarily tight. Kepler-78b sits less than one million miles from its star’s surface, and its orbital radius is approximately 40 times smaller than Mercury’s distance from the Sun. At that separation, the stellar surface fills a substantial fraction of the planet’s sky.

MIT physicist Saul Rappaport and colleagues calculated that for Kepler-78b to maintain that orbit without being immediately shredded by tidal forces, the planet would have to be composed almost entirely of iron or a very dense rocky material. Density measurements later confirmed a composition consistent with that constraint: using a two-component rock-iron model, the iron mass fraction of Kepler-78b is estimated at 32% ± 26%, overlapping with an Earth-like composition.

Mass, Radius, and Composition Confirmed in 2013

On October 30, 2013, two independent research groups published simultaneous papers in the journal Nature establishing Kepler-78b’s mass and radius with enough precision to determine its bulk density. At that time it was the smallest exoplanet for which both mass and radius were accurately known, and the exoplanet most similar to Earth in mass, radius, and mean density. It was also the first Earth-sized exoplanet confirmed to have a rocky, Earth-like composition.

The mass measurement required radial-velocity spectroscopy — detecting the tiny wobble the planet induces in the star’s spectrum. The HARPS-N spectrograph, which achieves a radial-velocity precision of 1.2 m/s in a one-hour exposure on a slowly rotating late-G or K-type dwarf star, was the instrument that made this possible. That precision of 1.2 m/s is roughly half the expected radial-velocity semi-amplitude of the planet’s signal, meaning the detection sat at the very edge of what the instrument could accomplish.

The Formation Problem

Despite everything known about Kepler-78b’s composition and orbit, planetary science has no accepted explanation for how it got there. Current theories of planet formation rule out formation in place: when Kepler-78 was young it was larger, and Kepler-78b’s present orbit would have sat inside the star. The planet therefore could not have accreted at its current location.

Migration from a more distant birthplace is equally problematic. The planet could not have migrated from further out without spiraling all the way into the star; no mechanism is known that could have halted Kepler-78b at its present distance. Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics described Kepler-78b as “an abomination” to planetary formation theory — a characterization that accurately reflects the state of the field: no known process produces it.

A 2019 study by Strugarek and colleagues modeled the corona of Kepler-78 using its Zeeman-Doppler Imaging magnetic map and found that the extreme proximity of the planet allows star-planet magnetic interaction to channel energy flux densities of up to a few kW m⁻² toward the central star. This magnetic coupling adds a further layer of physical complexity to a system that already sits outside the boundaries of standard formation models.

Eventual Fate

Gravitational tidal forces are drawing Kepler-78b progressively closer to its host star. Theorists predict the planet will be torn apart and swallowed by Kepler-78 within approximately three billion years. The tidal inspiral is a consequence of the same orbital proximity that makes the system so unusual: at less than one million miles from the stellar surface, the difference in gravitational pull between the planet’s near and far sides is large enough to dissipate orbital energy steadily over geological timescales.

Did you know

When Kepler-78b first appeared in the Kepler Eclipsing Binary catalog in 2011, automated analysis flagged it as an eclipsing binary star with a period of 0.71 days — exactly twice the planet’s true orbital period of 0.355 days. The factor-of-two error delayed recognition of its planetary nature. (arxiv.org)
Kepler-78b completes one full orbit in about 8.5 hours. A person finishing a standard 8.5-hour workday or waking from a night’s sleep has waited exactly one Kepler-78b year. (news.mit.edu)
Astronomer Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics called Kepler-78b “an abomination” to planetary formation theory. No known mechanism can explain how a rocky planet formed or migrated to an orbit that, during the star’s early inflated phase, would have placed the planet inside the star itself. (sciencedaily.com)
The HARPS-N spectrograph used to measure Kepler-78b’s mass achieves a radial-velocity precision of 1.2 m/s in a one-hour exposure on a slowly rotating late-G or K-type dwarf — roughly half the expected radial-velocity semi-amplitude of Kepler-78b’s signal. The planet was barely within the instrument’s reach. (arxiv.org)
The discovery team recovered Kepler-78b’s ultra-short-period signal using a Fourier Transform method. The standard Box-fitting Least Squares algorithm in the main Kepler pipeline missed it entirely, which is why the planet was not flagged during routine data processing. (arxiv.org)

Data sources

Data updated: 2026-10-30