KOI-2700b

KOI-2700b is a confirmed exoplanet orbiting the K-type main-sequence star KIC 8639908, roughly 1,608 light-years away in Lyra. It completes one orbit every 0.91 days at a distance of just 0.0150 AU. Its small mass and extreme proximity to the star cause it to evaporate, shedding material that forms a comet-like dust tail stretching about a quarter of its orbit.

The dust tail is the planet’s most striking feature. It is made of material escaping the surface, much like a comet’s, and offers a rare window into exoplanet composition and formation. The tail likely consists of fayalite (Fe₂SiO₄) and/or corundum (Al₂O₃); pure iron, graphite, or silicon carbide have been ruled out. This tail leaves a clear signature in the star’s light curves: transit dips vary in depth from one transit to the next, are asymmetrical (falling sharply then recovering gradually), and the star brightens slightly just before transit due to forward scattering by dust grains. Additionally, a cloud of partially ionized sodium vapor may surround the planet, extending to a size comparable to the host star (about 0.54 solar radii).

The planet’s exact mass and radius are unknown, but it is expected to be very small and rocky. A mass of less than about 0.03 Earth masses is needed to produce detectable dust, and planets larger than roughly half Earth’s radius likely do not emit a dust tail at all. The discovery paper notes that modeled upper limits for mass (0.86 Earth masses) and radius (1.06 Earth radii) are probably far too high; the planet may be closer to the Moon in size (0.27 Earth radii). Further research suggests its true radius likely lies between 0.1 and 0.3 Earth radii, smaller than Mercury (0.36 Earth radii). Mass loss is estimated at roughly 2 lunar masses (0.0246 Earth masses) per billion years—about 6,000 metric tons per second—and not below 0.007 Earth masses per billion years.

The host star, KIC 8639908, is a faint 15th-magnitude K5 main-sequence star in Lyra. It has a mass of 0.546 solar masses, a radius of 0.540 solar radii, an effective temperature of 4,296 K, and radiates 8.9% of the Sun’s luminosity. It is very metal-poor, with a metallicity of −0.7 (one-fifth the Sun’s iron content). Similarities have been noted between this system and Kepler-1520, a K4V star hosting another disintegrating exoplanet with a comet-like tail.