Beta Pictoris

A Potential Exomoon From The Predicted Planet Obliquity Of β Pictoris b

By Keith Cowing

Status Report

astro-ph.EP

December 17, 2024

Filed under astro-ph.EP, astronomy, beta Pictoris b, Exomoon, exoplanet, imaging, telescope, β Pictoris b

A Potential Exomoon From The Predicted Planet Obliquity Of β Pictoris b — Schematic diagram (not to scale) of four angular momentum vectors in the β Pictoris system: total angular momentum (Lˆ), orbital angular momentum of planets b and c (nˆorbit,b, nˆorbit,c), and the spin vector of planet b (nˆspin,b). The mutual inclination between nˆorbit,b and nˆorbit,c is ∼ 1 ◦. Secular spin-orbit resonance can occur when the period of orbit nodal regression (path along grey dotted circle on the left) has a similar period to the spin axis precession (path along black dotted circle on the right). The period of spin axis precession can be greatly shortened in the presence of a close-in equatorial exomoon, which acts as a lever and effectively extends the planet’s equatorial bulge. I is the orbital inclination of β Pictoris b (orange ellipse) relative to the invariable plane (grey ellipse). — astro-ph.EP

Planet obliquity is the alignment or misalignment of a planet spin axis relative to its orbit normal. In a multiplanet system, this obliquity is a valuable signature of planet formation and evolutionary history.

The young β Pictoris system hosts two coplanar super-Jupiters and upcoming JWST observations of this system will constrain the obliquity of the outer planet, β Pictoris b. This will be the first planet obliquity measurement in an extrasolar, multiplanet system. First, we show that this new planet obliquity is likely misaligned by using a wide range of simulated observations in combination with published measurements of the system.

Motivated by current explanations for the tilted planet obliquities in the Solar System, we consider collisions and secular spin-orbit resonances. While collisions are unlikely to occur, secular spin-orbit resonance modified by the presence of an exomoon around the outer planet can excite a large obliquity.

The largest induced obliquities (∼60∘) occur for moons with at least a Neptune-mass and a semimajor axis of 0.03−0.05 au (40−70 planet radii). For certain orbital alignments, such a moon may observably transit the planet (transit depth of 3−7%, orbital period of 3−7 weeks).

Thus, a nonzero obliquity detection of β Pictoris b implies that it may host a large exomoon. Although we focus on the β Pictoris system, the idea that the presence of exomoons can excite high obliquities is very general and applicable to other exoplanetary systems.

Michael Poon, Hanno Rein, Dang Pham

Comments: 10 pages, 8 figures, accepted for publication in The Open Journal of Astrophysics
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2412.05988 [astro-ph.EP] (or arXiv:2412.05988v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2412.05988
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Journal reference: The Open Journal of Astrophysics Volume 7, 10th December 2024
Related DOI:
https://doi.org/10.33232/001c.127130
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Submission history
From: Michael Poon
[v1] Sun, 8 Dec 2024 16:33:55 UTC (1,131 KB)
https://arxiv.org/abs/2412.05988
Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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