HD 106906 b

Lore & Background

The star HD 106906 is a spectroscopic binary composed of two F5V main-sequence stars with a combined mass of about 2.6 solar masses, and the system is estimated to be about 13 million years old. It is a likely member of the Scorpius–Centaurus association and is surrounded by a debris disk oriented 21 degrees away from the companion, extending from about 65 AU to asymmetrically between 120 and 550 AU. The companion was discovered on December 4, 2013, by University of Arizona graduate student Vanessa Bailey and an international team, using the Magellan Telescopes at Las Campanas Observatory in Chile. The discovery was first published as a preprint on arXiv and later in The Astrophysical Journal Letters.

The companion's high surface temperature (around 1800 K) is considered a relic of its recent formation. Its mass depends on formation assumptions: either a hot-start or cold-start model. The discovery team evaluated the possibility that HD 106906 b is not gravitationally bound but a chance alignment, finding the odds of such a coincidence to be less than 0.01%. The first orbit was determined in 2020, finding a semi-major axis of 850 AU, an orbital period of 15,000 years, an eccentricity of 0.44, and an orbital inclination relative to the debris disk of 36 or 40 degrees.

Reader's Guide

HD 106906 b is significant because its extreme separation from its host star—about 738 AU—poses a puzzle for planet formation theories. In-situ formation from a protoplanetary disk is considered impossible at such a distance, as disks are not thought to be extensive enough. The discovery team and other astronomers have proposed several formation mechanisms: formation as part of a binary system (though the mass ratio of ~140:1 is far outside the typical 10:1 for binary stars), or formation closer to the star followed by gravitational scattering by another object.

The scattering hypothesis is supported by Hubble Space Telescope images showing a highly asymmetric debris disk beyond 200 AU, suggesting a dynamical upheaval involving the planet and another perturber, such as a second planet or a passing star. An analysis of 461 nearby stars using Gaia observations identified two stars that passed within 1 parsec of the system between 2 and 3 million years ago, but their closest passage was likely too distant to significantly affect the disk or companion. Polarimetric observations in 2021 found the debris disk is unusually eccentric, and planet-disk interactions are likely to blame, suggesting HD 106906 b likely formed in an inner orbit and was ejected.

A 2021 study found the planet's rotational axis is misaligned with both its orbit and possibly the circumstellar disk, pointing to formation through gravitational instability, similar to star formation. A 2023 publication rejected in-situ formation because the planet's inclined orbit would introduce a warp in the debris disk that is not observed, proposing instead an encounter with a rogue planet. Archival spectral data found a carbon-to-oxygen ratio consistent with the stellar association, supporting a stellar-like formation and making it unlikely that it formed like a planet in a disk around the binary.