On the Detection of Exomoons Transiting Isolated Planetary-Mass Objects

TL;DR

This paper explores the potential for detecting transiting exomoons around isolated planetary-mass objects (IPMOs) using current and upcoming telescopes, highlighting the feasibility and challenges involved.

Contribution

It introduces the concept that transiting exomoons around IPMOs are detectable with existing instruments and provides a proof-of-concept using archival data.

Findings

Transiting moons are expected around 10-15% of IPMOs.

Single transit observations with JWST could detect moons like Io or Titan.

Archival Spitzer data shows a candidate event possibly caused by an exomoon.

Abstract

All-sky imaging surveys have identified several dozen isolated planetary-mass objects (IPMOs), far away from any star. Here, we examine the prospects for detecting transiting moons around these objects. We expect transiting moons to be common, occurring around 10-15% of IPMOs, given that close-orbiting moons have a high geometric transit probability and are expected to be a common outcome of giant planet formation. IPMOs offer an advantage over other directly imaged planets in that high-contrast imaging is not necessary to detect the photometric transit signal. For at least 30 (>50%) of the currently known IPMOs, observations of a single transit with the James Webb Space Telescope would have low enough forecasted noise levels to allow for the detection of an Io-like or Titan-like moon. Intrinsic variability of the IPMOs will be an obstacle. Using archival time-series photometry of IPMOs with the Spitzer Space Telescope as a proof-of-concept, we found evidence for a fading event of 2MASS J1119-1137 AB that might have been caused by intrinsic variability, but is also consistent with a single transit of a habitable-zone 1.7$R_\oplus$ exomoon. Although the interpretation of this particular event is inconclusive, the characteristics of the data and the candidate signal suggest that Earth-sized habitable-zone exomoons around IPMOs are detectable with existing instrumentation.