Rogue planets
Interstellar planetary-mass objects not bound to any star.
A rogue planet, also termed a free-floating planet (FFP) or an isolated planetary-mass object (iPMO), is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf. Rogue planets may originate from planetary systems in which they are formed and later ejected, or they can also form on their own, outside a planetary system. The Milky Way alone may have billions to trillions of rogue planets, a range the upcoming Nancy Grace Roman Space Telescope is expected to refine.
- discovery teams
- UK team P. W. Lucas & P. F. Roche (Orion Nebula); Spanish team María Rosa Zapatero Osorio et al. (σ Orionis cluster)
- detection methods
- direct imaging and microlensing
- estimated abundance
- nearly two Jupiter-mass rogue planets per star (2011 study); upper limit 0.25 per main-sequence star (2017 study)
- largest known group
- at least 70, up to 170, discovered as of December 2021
Lore & Background
W. Lucas & P. F. Roche using UKIRT in the Orion Nebula, and María Rosa Zapatero Osorio et al. using Keck spectroscopy in the σ Orionis cluster. The terminology varies: astronomical papers often use 'isolated planetary-mass object' or 'free-floating planet,' while press releases may use 'rogue planet,' 'starless planet,' or 'wandering planet.'
Reader's Guide
Rogue planets are significant because they challenge traditional definitions of planets and star formation. They may form either like stars (via gravitational collapse) or like planets (in protoplanetary disks, later ejected). The discovery of Jupiter-mass binary objects (JuMBOs) in the Orion Nebula, making up at least 9% of iPMOs, raises questions about formation mechanisms—whether they formed in situ like stars or were ejected as binaries without breaking apart. Microlensing studies, such as the 2011 survey of 50 million stars, estimated nearly two Jupiter-mass rogue planets per star, though a 2017 study with larger statistics reduced that upper limit to 0.25 per main-sequence star.
Did You Know?
- The largest group of rogue planets discovered as of December 2021 numbers at least 70, found in the OB association between Upper Scorpius and Ophiuchus.
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