20 Leonis Minoris

Lore & Background

20 Leonis Minoris is a binary star system located 48.7 light-years from the Sun, based on a parallax shift of 66.996 mas. It has a relatively high proper motion and is moving away from the Sun with a radial velocity of +56 km/s. The system made its closest approach about 150,000 years ago, coming within 32.2 light-years (9.86 pc). The primary star is a G-type main-sequence star with a classification of , having 12% more mass and a 25% larger radius than the Sun. It is about seven billion years old and spins with a rotation period of 10.6 days. The companion is an active red dwarf star with relatively high metallicity. The two stars are currently separated by 14.5 arc seconds, corresponding to a projected separation of 2016 AU. A candidate exoplanet orbiting the primary star (HD 86728) was first detected in 2020 and confirmed in 2024 as the first discovery of the NEID Earth Twin Survey (NETS). With a minimum mass of 9.2 Earth masses and an orbital period of 31 days, it is most likely a hot Neptune-type planet.

Reader's Guide

The significance of 20 Leonis Minoris lies in its confirmed exoplanet, which represents the first discovery of the NEID Earth Twin Survey (NETS). This detection, initially made in 2020 and confirmed in 2024, highlights the system's role in advancing exoplanet detection techniques. The planet, with a minimum mass of 9.2 Earth masses and a 31-day orbit, is classified as a hot Neptune-type, providing insights into planetary formation and evolution around Sun-like stars. The binary nature of the system, with a G-type primary and an active red dwarf companion separated by 2016 AU, offers a context for studying planet formation in multiple-star environments. The primary star's age of about seven billion years and its relatively high metallicity companion further contribute to understanding stellar and planetary system development. The system's close approach to the Sun about 150,000 years ago adds historical interest to its dynamics.

Did You Know?

• 20 Leonis Minoris is located 48.7 light-years from the Sun.
• The system hosts a confirmed exoplanet with a minimum mass of 9.2 Earth masses.

Stellar Nature and Physical Profile

21 Leonis Minoris is a white main-sequence star of spectral type A7V, ranking as the third-brightest object in its constellation with an average apparent magnitude of 4.5. Located roughly 98 light-years (30 parsecs) from Earth, it shines with about ten times the luminosity of our Sun, a remarkable output for a star still on the main sequence. What distinguishes it most dramatically is its rotation: it spins on its axis in under twelve hours, a pace so extreme that the star is very likely flattened at its poles. This rapid spin sets it apart from the slower-turning giants that dominate the brighter ranks of Leo Minor, such as the orange giant 46 Leonis Minoris or the binary system of Beta Leonis Minoris. As a relatively young, hot, and fast-rotating A-type star, 21 Leonis Minoris represents a different evolutionary stage entirely from its brighter neighbours, offering astronomers a window into the physics of rapidly rotating stellar bodies still fusing hydrogen in their cores.

A Star in a Constellation Without Ancient Roots

Leo Minor, the small and faint constellation that claims 21 Leonis Minoris as its third-brightest member, has no pedigree in classical Greek or Roman astronomy. Aratus and Ptolemy both noted the region as undefined, with Ptolemy specifically classifying its stars as amorphōtoi—belonging to no constellation outline—and simply lumping them into the larger Leo. It was not until 1687 that Johannes Hevelius, working on his star atlas Firmamentum Sobiescianum, carved out ten new constellations and gave this patch of sky the Latin name meaning "the smaller lion," a deliberate nod to its beastly neighbours the Lion and the Great Bear. The constellation is compact, spanning just 232.0 square degrees and ranking 64th among the 88 IAU-recognized constellations. Only 37 stars in the entire figure exceed magnitude 6.5, and a mere three are brighter than 4.5. Within this sparse stellar population, 21 Leonis Minoris at magnitude 4.5 holds a modest but notable position as the third-brightest, trailing only the orange giant 46 Leonis Minoris and the binary Beta Leonis Minoris.

Pulsations and the Science of Standard Candles

Beyond its rapid rotation, 21 Leonis Minoris belongs to a class of variable stars known as Delta Scuti variables, a designation that speaks to its rhythmic, short-period pulsations. These stars oscillate with periods of at most six hours, making them among the fastest pulsators observed in the night sky. Their regular, predictable brightness variations have made them invaluable to astronomers as standard candles—objects whose intrinsic luminosity can be estimated from their pulsation properties, allowing distance measurements to otherwise faint stars. They also serve as natural laboratories for asteroseismology, the study of stellar interiors through the analysis of oscillation modes, much as seismologists use earthquakes to probe Earth's core. For a star sitting roughly 98 light-years away and shining at about ten solar luminosities, 21 Leonis Minoris thus plays a dual role: it is both a subject of fundamental stellar physics and a practical tool in the astronomer's distance-measuring toolkit, all while spinning so quickly that its shape is very likely flattened.

Finding It in the Northern Sky

Observing 21 Leonis Minoris requires patience and dark skies. At an average apparent magnitude of 4.5, it is visible to the unaided eye under good conditions, but it sits within a constellation that Patrick Moore famously described as having "dubious claims to a separate identity." Leo Minor occupies a small triangular patch of the northern celestial hemisphere, bordered by Ursa Major to the north, Lynx to the west, Leo to the south, and grazing the corner of Cancer to the southwest. Its official 16-sided boundary, fixed by Eugène Delporte in 1930, spans right ascension from 9h 22.4m to 11h 06.5m and declination from 22.84° to 41.43°. The constellation reaches its highest point in the sky at midnight on 24 February and at 9 p.m. on 24 May, making late winter and early spring the prime windows for spotting its faint triangular asterism. The IAU's three-letter abbreviation for the region is LMi, a compact label for a sky area that covers just 0.562 per cent of the total celestial sphere.