Kepler-1638b

Lore & Background

Kepler-1638b was identified as a super-Earth with a radius and a predicted mass of 4.16 Earths, orbiting within its star's habitable zone at 0.745 AU with a period of 259.3365 days. Its host star, Kepler-1638, is a G-type subgiant located about 5,000 light years away in Cygnus. The planet was confirmed in 2016 as part of a statistical validation of hundreds of Kepler candidates.

However, the initial parameters were estimated before the first measurement of the host star's parallax from Gaia DR2 in 2018. The Gaia parallax indicated a distance of about 1,548 parsecs, much farther than the pre-Gaia estimate of about 764 parsecs. This revised distance led to a significantly larger estimate of the star's radius and, consequently, the planet's radius. A 2018 study found a planetary radius that would make the planet an ice giant like Neptune, and thus not potentially habitable in an Earth-like sense.

As of January 2021, Kepler-1638 is the farthest star with a known exoplanet orbiting in the habitable zone, though the planet's own habitability remains disputed due to the revised size estimate.

Reader's Guide

Kepler-1638b's significance lies in its role as a cautionary example of how revised stellar distances can dramatically alter the interpretation of exoplanet properties. Initially hailed as a potentially habitable super-Earth, the planet's status was upended by Gaia DR2 parallax data, which showed the star to be much farther than previously thought. This revision increased the inferred radius of both star and planet, transforming the world from a rocky super-Earth into an ice giant more akin to Neptune. The case underscores the importance of accurate astrometry in exoplanet characterization and the provisional nature of habitability assessments. Despite the revised classification, Kepler-1638b remains the most distant known exoplanet orbiting within its star's habitable zone, a record that highlights the vast distances over which Kepler's transit survey operated. Its legacy is a reminder that early estimates of exoplanet parameters can be subject to large uncertainties, and that the definition of 'potentially habitable' must be treated with caution until robust stellar distances are available. The planet also illustrates the dynamic nature of exoplanet science, where new data can fundamentally change our understanding of a world's nature and potential for life.

Did You Know?

• The planet's predicted mass is 4.16 Earths.
• Gaia DR2 parallax data revised the star's distance from about 764 pc to about 1,548 pc, changing the planet's classification from super-Earth to ice giant.