BD+20 594b
A sub-Neptune exoplanet discovered by Kepler and HARPS.
BD+20°594b, also called K2-56b, is a sub-Neptune exoplanet. It was discovered on January 28, 2016, by astrophysicist Néstor Espinoza and his team at the Catholic University of Chile, using data from the two-wheeled Kepler mission (K2) and the HARPS spectrometer at La Silla in Chile. The planet orbits a K-type star located 496.08 light-years away in the constellation Taurus.
Its name comes from the Bonner Durchmusterung catalogue: BD+20°594b means it is the first planet found around star BD+20°594, the 594th entry in the +20-degree declination zone. The alternative name K2-56b indicates it is the first planet orbiting the 56th star catalogued in the extended K2 Kepler mission.
BD+20°594b is substantially smaller than Neptune, with a radius and mass that classify it as a sub-Neptune. Early estimates suggested a mass large enough to give it an unusually high density compatible with a rocky composition, possibly making it a mega-Earth. (Planets with a radius greater than 1.6 times Earth’s are generally not considered rocky.) However, research published in 2026 found a significantly lower mass and density.
Quick Facts
- Discoverer
- Néstor Espinoza, et al.
- Discovery Site
- Kepler space telescope / HARPS (La Silla, Chile)
- Discovered
- 2016
- Discovery Method
- Transit
- Apsis
- astron
- Semimajor
- 0.2226 · 0.0002
- Eccentricity
- 0
- Star
- BD+20°594
Facts from the source article.
Lore & Background
BD+20°594b was discovered on January 28, 2016 by astrophysicist Néstor Espinoza and his team from the Catholic University of Chile, using data from the two-wheeled Kepler mission (K2). The planet's designation BD+20 594b indicates it circles a star from the Bonner Durchmusterung catalogue, the 594th entry in the +20-degree zone, and that it is the first planet discovered orbiting that star. Its alternative designation K2-56b indicates it circles a star catalogued as part of the extended K2 Kepler mission, the 56th one, and that it is the first planet discovered orbiting that star.
With a radius and mass substantially smaller than Neptune, BD+20 594b is classified as a sub-Neptune. It was originally estimated to have a mass as large as a value that would give it an unusually high density compatible with a rocky composition, classifying it as a possible mega-Earth. It is believed that planets with a radius greater than 1.6 times the Earth's are not usually rocky. However, subsequent research published in 2026 found a significantly lower mass and density.
Reader's Guide
BD+20°594b is notable as a sub-Neptune exoplanet whose initial mass estimate suggested an unusually high density, leading to its classification as a possible mega-Earth. This raised questions about the boundary between rocky and gaseous planets, as it is believed that planets with a radius greater than 1.6 times Earth's are not usually rocky. The subsequent revision of its mass and density in 2026, finding significantly lower values, underscores the evolving understanding of exoplanet compositions and the importance of follow-up observations. Discovered through a collaboration between the Kepler space telescope and the HARPS spectrometer, the planet exemplifies the synergy between space-based transit detection and ground-based radial velocity measurements. Its dual naming—from the Bonner Durchmusterung catalogue and the K2 mission—reflects the multiple astronomical catalogs that track celestial objects. Located 496.08 light years away in Taurus, BD+20°594b contributes to the study of sub-Neptune planets and the diversity of planetary systems around K-type stars.
Did You Know?
- BD+20°594b is also known as K2-56b.
- It was discovered on January 28, 2016 by Néstor Espinoza and his team.
- The planet was originally estimated to have a mass large enough to suggest a rocky composition, classifying it as a possible mega-Earth.
Frequently Asked Questions
What is BD+20 594b?
BD+20 594b, also known as K2-56b, is a sub-Neptune-class exoplanet that circles a K-type star roughly 496 light-years from Earth. It sits in the constellation Taurus and was first identified in early 2016.
How was BD+20 594b discovered?
Néstor Espinoza and his colleagues at the Catholic University of Chile announced the planet's existence on January 28, 2016. They combined transit photometry from the K2 space telescope with radial-velocity measurements taken by the HARPS spectrograph at La Silla Observatory in Chile.
What does the name 'BD+20 594b' actually mean?
The designation traces back to the Bonner Durchmusterung star catalogue, where 'BD+20°594' identifies the host star as the 594th entry in the +20-degree declination band, and the trailing 'b' marks it as the first planet found around that star. The alternate label K2-56b simply reflects its discovery through the Kepler K2 mission.
How far away is BD+20 594b and where can I find it in the sky?
The system lies about 496.08 light-years out in the constellation Taurus. Because of that distance, the planet is far beyond the reach of any current ground-based or space-based telescope for direct imaging.
Why does BD+20 594b matter to exoplanet researchers?
As a sub-Neptune, it helps astronomers fill the size gap between super-Earths and true gas giants, a category that is strikingly absent from our own solar system. Its characterization through both transit and spectroscopic data makes it a useful reference point for modeling planetary atmospheres and orbital dynamics around cooler K-type stars.
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