(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. It was numbered by the Minor Planet Center on 18 May 2019, but as of 2025 it has not been named.

The object is in a 1:6 resonance with Neptune, meaning it completes roughly one orbit for every six orbits of Neptune. Its orbit has a semi-major axis of 99.89 AU, an eccentricity of 0.58, and an inclination of 21° to the ecliptic. It ranges from 42.3 AU at perihelion to 157.5 AU at aphelion, and takes 998 years and 4 months to orbit the Sun. It came to perihelion in 1954 and is currently 60.9 AU from the Sun.

Physical size estimates vary. Johnston's Archive gives a diameter of about 536 km assuming an absolute magnitude of 4.4 and an albedo of 0.126. Mike Brown estimates 549 km using a magnitude of 4.6 and albedo 0.09. 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 range from 345 to 773 km.

Reader's Guide

As a 1:6 resonant trans-Neptunian object, 2008 ST291 is part of a rare dynamical class that helps constrain models of Neptune's migration and the early Solar System. Its large estimated size—potentially over 500 km—places it among the larger known TNOs, though estimates vary due to uncertain albedo. The lack of precovery images and its recent numbering in 2019 highlight the ongoing process of cataloging distant objects. Its discovery by a team including Michael Brown, known for finding many large TNOs, adds to its context. The object's orbit, taking nearly a millennium to complete, and its current distance of 60.9 AU from the Sun, underscore the vast timescales and distances involved in studying the outermost Solar System. The range of diameter estimates from different sources reflects the inherent uncertainties in characterizing such distant bodies.

Did You Know?

• It was discovered on 24 September 2008 at Palomar Observatory.
• Its estimated diameter ranges from 345 to 773 km depending on assumed albedo.