(528381) 2008 ST291

Lore & Background

2008 ST291 was discovered on 24 September 2008 by American astronomers Megan Schwamb, Michael Brown and David Rabinowitz at the Palomar Observatory in California. No earlier precovery images are known. The Minor Planet Center numbered it on 18 May 2019, but as of 2025 it has not been named.

The object is locked in a 1:6 resonance with Neptune, meaning it completes roughly one orbit for every six orbits Neptune makes. Its orbit has a semi-major axis of 99.89 AU, an eccentricity of 0.58, and an inclination of 21° with respect to the ecliptic. It ranges from 42.3 AU at perihelion to 157.5 AU at aphelion, and it last came to perihelion in 1954. Currently it is located 60.9 AU from the Sun.

Physical size estimates vary depending on assumed albedo and absolute magnitude. Johnston's Archive gives a diameter of about 536 km assuming an albedo of 0.126. Mike Brown estimates 549 km using an albedo of 0.09 and a magnitude of 4.6. The Asteroid Dynamic Site records a brighter magnitude of 4.3, yielding 612 km with the same albedo formula. Using the average of these magnitudes and a standard albedo range of 0.25–0.05, possible sizes span 345–773 km.

Reader's Guide

2008 ST291 is significant as a 1:6 resonant trans-Neptunian object, a class that helps astronomers understand the dynamical structure of the outer Solar System and the influence of Neptune's migration. Its large estimated size—potentially over 500 km—places it among the larger known objects in the Kuiper belt and scattered disk, though its exact diameter remains uncertain due to differing albedo assumptions. The object's orbit, taking nearly a thousand years to complete, and its current distance of 60.9 AU from the Sun, highlight the vast timescales and distances involved in studying the outermost Solar System. Its discovery by a team including prominent astronomers Megan Schwamb and Michael Brown adds to the catalog of distant bodies that inform models of planetary formation and evolution. The lack of a name as of 2025 reflects the slow pace of naming for such remote objects. Ongoing observations and future studies may refine its size, orbit, and physical properties, contributing to a clearer picture of the trans-Neptunian region.

Did You Know?

• 2008 ST291 is a 1:6 resonant trans-Neptunian object, meaning it orbits the Sun once for every six orbits of Neptune.
• It was discovered on 24 September 2008 by American astronomers Megan Schwamb, Michael Brown and David Rabinowitz at Palomar Observatory.
• Its orbit takes 998 years and 4 months, with a semi-major axis of 99.89 AU.
• Estimated diameter ranges from 345 to 773 km, depending on assumed albedo and magnitude.