Kepler-277c

Lore & Background

Kepler-277c was detected using the transit method and transit-timing variations, which allowed both its mass and radius to be determined. Its radius is close to that of Neptune, yet its mass is extremely high—about 64.2 times Earth's mass, approaching Saturn's mass of 95.16 Earth masses. This yields a density of 9.33 g/cm³ and a surface gravity 5.7 times that of Earth. Such a high density for an object of this size implies that Kepler-277c is a massive rocky planet with a small fraction of its mass as water, rather than a gaseous mini-Neptune.

The planet orbits its host star at an average distance of 0.209 AU, completing one orbit every 33.006 days. At this distance, it is very hot, with an equilibrium temperature of about 745 K—hot enough to melt certain metals. It is likely tidally locked to its star and lies close to a 1:2 orbital resonance with its sister planet Kepler-277b, which orbits at 0.136 AU.

The host star Kepler-277 is a large yellow star with a radius of 1.69 solar radii, a mass of 1.12 solar masses, a temperature of 5946 K, and a metallicity of -0.315 [Fe/H]. Its age is unknown. The star's large radius relative to its mass and temperature suggests it may be a subgiant star.

Reader's Guide

Kepler-277c is significant as one of the most massive and largest rocky planets ever discovered, challenging the conventional expectation that planets of Neptune's size are gaseous. Its high density indicates a predominantly rocky composition with some water, placing it in the rare category of 'mega-Earths.' Along with Kepler-277b, its mass was determined through transit-timing variations, a technique that measures gravitational interactions between planets. The planet's proximity to its star and high equilibrium temperature suggest a harsh environment, likely tidally locked. Its legacy lies in expanding the known diversity of exoplanet compositions, showing that rocky planets can exist at sizes and masses previously thought impossible for solid bodies. The system also provides a laboratory for studying orbital resonances and planetary formation in compact multi-planet systems.

Did You Know?

• Kepler-277c is the third most massive and second-largest rocky planet ever discovered.
• Its mass of about 64.2 Earth masses approaches that of Saturn (95.16 Earth masses).
• The planet's density of 9.33 g/cm³ implies a composition mainly of rock with some water.
• It orbits its host star every 33.006 days at a distance of 0.209 AU.

Discovery and the TTV Revelation

In 2014, the Kepler Space Telescope added another remarkable entry to the growing catalogue of distant worlds by identifying the Kepler-277 system, a pair of rocky planets orbiting a yellow star. Kepler-277c was not spotted through a single clean dip in starlight; rather, its mass was extracted through transit-timing variations, a technique that reads the gravitational tugs one planet exerts on another. Because the two worlds perturb each other's orbital periods, the small irregularities in their transit times reveal masses that a simple transit alone cannot provide. This method allowed astronomers to pin down the mass of both Kepler-277b and its larger sister, Kepler-277c, without ever seeing the planets directly. The discovery confirmed that the Kepler mission could still surprise researchers years into its operation, yielding a system where both planets push the boundaries of what a rocky world can be in terms of sheer size and mass.

The Largest Rocky World by Radius

Among all confirmed exoplanets, Kepler-277c holds the distinction of being the largest terrestrial planet ever identified by radius. It sits at the top of the rocky-planet size ranking, ahead of both PSR J1719−1438 b and its own sister Kepler-277b, which is the third-largest by radius and the second-most-massive rocky world known. The pairing is extraordinary: two planets in the same system that together occupy the top three slots in the rocky-planet size chart. While PSR J1719−1438 b edges out both in mass, Kepler-277c's radius places it in a class of its own. These objects are sometimes grouped under the informal label 'mega-Earth,' a category that underscores just how far beyond our own planet their dimensions stretch. For Kepler-277c, the fact that it is the widest rocky world yet found makes it a benchmark against which future discoveries will be measured.

Orbital Architecture and Near-Resonance

Kepler-277c circles its host star at a mean distance of roughly 0.209 astronomical units, a comfortable outer lane compared to its inner companion Kepler-277b, which completes an orbit every 17.324 days at about 0.136 AU. The two planets sit close to a 2:1 mean-motion resonance, meaning that for every two laps the inner world completes, the outer one finishes approximately one. This near-commensurability is not merely a curiosity; it is the very mechanism that amplifies the transit-timing variations used to measure both planets' masses. Gravitational interactions at a resonant angle produce periodic, measurable shifts in transit times, turning what would be a subtle signal into a robust mass determination. The resonance also hints at a dynamically settled system, where the two rocky worlds have coexisted for a long stretch of orbital history without colliding or being ejected, a stability that makes the Kepler-277 pair a useful case study for multi-planet architecture.

The Host Star and System Context

Kepler-277, the star anchoring this two-planet system, is a large yellow star with a radius of 1.69 times that of the Sun and a mass of 1.12 solar masses. Its effective temperature of 5,946 K is modestly higher than the Sun's 5,778 K, and its metallicity, measured at −0.315 in [Fe/H], is notably below the solar value of 0.00. The star's age remains unknown, though its unusually large radius relative to its mass and temperature has led researchers to suggest it may be a subgiant, a star that has begun evolving off the main sequence. The system's lower-than-solar metallicity is a point of interest, since the formation of massive rocky planets in metal-poor environments challenges some standard models of core accretion. Together, these stellar parameters frame the backdrop against which two of the most extreme rocky planets ever found continue their close, resonant dance.