K2-288Bb

Lore & Background

K2-288 was observed by the Kepler space telescope during Campaign 4 of its extended K2 mission from April through September 2015. Initial analysis of the data found only two transits, insufficient for follow-up, and the system was set aside. After improved modeling of systematic errors, the data was uploaded to the Zooniverse project Exoplanet Explorers in April 2017. Citizen scientists spotted three transits of the red dwarf star EPIC 210693462 and began a lengthy discussion, noting the planet candidate's similarity in size and temperature to Earth. This drew the attention of the original astronomers and a team at NASA Goddard, who independently found the same transits and initiated follow-up observations.

Spectra from the Keck Observatory revealed a secondary companion star, raising the possibility that the transit signal could be a false positive. However, the team concluded it was far more likely to be a real exoplanet. Using data from Kepler and a transit observed by the Spitzer Space Telescope, they determined the planet transits the smaller, secondary star. The planet's radius, orbit, and temperature were then calculated, and the results were announced at the 233rd American Astronomical Society meeting in Seattle on 7 January 2019.

K2-288Bb has an equilibrium temperature of 226.36 K and receives less sunlight than Earth. It orbits every 31.393 days at a distance of about 0.164 AU, well within the habitable zone due to the small size of its host star. The planet is probably tidally locked, with one side permanently facing the star. Its mass is unknown, and it could be either a low-density mini-Neptune or a large rocky super-Earth. The host binary consists of two red dwarfs: K2-288A (52% solar mass, 45% solar radius) and K2-288B (33% solar mass, 32% solar radius), orbiting each other at about 55 AU.

Reader's Guide

K2-288Bb is significant as the third transiting planet system identified by the Exoplanet Explorers program, following K2-138 and K2-233, demonstrating the value of citizen science in exoplanet discovery. Its radius of 1.90 Earth radii places it squarely within the Fulton gap—a sparsely populated size range between 1.5 and 2.0 Earth radii where planets are uncommon and their composition is poorly understood. This makes K2-288Bb a key target for understanding the transition from rocky super-Earths to volatile-rich mini-Neptunes. The planet's location in the habitable zone of its host star, combined with its temperate equilibrium temperature of 226.36 K, raises the possibility that it could support liquid water if it has a suitable atmosphere. However, its composition remains unknown, and it could be either a potentially habitable rocky or water-rich world or a hostile gas planet. The binary nature of the system adds further complexity, as the planet orbits the smaller secondary star. Future radial velocity studies are needed to determine its mass and clarify its nature.

Did You Know?

• K2-288Bb was discovered by citizen scientists analyzing data from the Kepler space telescope's K2 mission through the Exoplanet Explorers Zooniverse project.
• Its radius of 1.90 Earth radii falls within the Fulton gap, a size range where rocky super-Earths transition into mini-Neptunes and where planets are uncommon.
• The planet orbits the smaller star in a binary system of two red dwarfs, which orbit each other at a distance of about 55 AU.
• K2-288Bb has an equilibrium temperature of 226.36 K, lower than Earth's 255 K, and receives less sunlight than Earth.