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Astronomers Make First Direct Mass Measurement of a Rogue Planet

Simultaneous Earth–Gaia observations produced a microlens‑parallax signal that broke the long‑standing mass–distance degeneracy.

Overview

  • The microlensing event KMT-2024-BLG-0792/OGLE-2024-BLG-0516 reveals a Saturn-class object with about 22% of Jupiter’s mass located roughly 3,000 parsecs in the Milky Way.
  • Ground-based surveys KMTNet and OGLE recorded the fleeting May 3, 2024 event, while ESA’s Gaia also captured it, enabling a crucial parallax measurement.
  • Gaia observed the target six times over 16 hours, producing a roughly two-hour timing offset from Earth that yielded the parallax needed to disentangle mass and distance.
  • Combining microlens parallax with finite‑source point‑lens modeling confirmed the object’s planetary nature and validated a method for characterizing free‑floating worlds.
  • The measured low mass supports an origin in a planetary system followed by ejection, and upcoming space surveys such as NASA’s Roman mission (planned for 2027), CSST and Earth 2.0 are expected to expand the sample dramatically.