Rogue planet
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.
[8] The two first discovery papers use the names isolated planetary-mass objects (iPMO)[9] and free-floating planets (FFP).
Isolated planetary-mass objects (iPMO) were first discovered in 2000 by the UK team Lucas & Roche with UKIRT in the Orion Nebula.
[10] In the same year the Spanish team Zapatero Osorio et al. discovered iPMOs with Keck spectroscopy in the σ Orionis cluster.
[28] In September 2020, astronomers using microlensing techniques reported the detection, for the first time, of an Earth-mass rogue planet (named OGLE-2016-BLG-1928) unbound to any star and free floating in the Milky Way galaxy.
Astronomers therefore turn to isolated planetary-mass objects (iPMO) that were found via the direct imaging method.
It is no surprise that the vast majority of iPMOs are found inside young nearby star-forming regions of which astronomers know their age.
For WISE 1534–1043[38] one alternative scenario explains this object as an ejected exoplanet due to its high Vtan of about 200 km/s, but its color suggests it is an old metal-poor brown dwarf.
Herschel far-infrared observations have shown that OTS 44 is surrounded by a disk of at least 10 Earth masses and thus could eventually form a mini planetary system.
Some large surveys include: As of December 2021, the largest-ever group of rogue planets was discovered, numbering at least 70 and up to 170 depending on the assumed age.
They are found in the OB association between Upper Scorpius and Ophiuchus with masses between 4 and 13 MJ and age around 3 to 10 million years, and were most likely formed by either gravitational collapse of gas clouds, or formation in a protoplanetary disk followed by ejection due to dynamical instabilities.
[46][16][50][18] Follow-up observations with spectroscopy from the Subaru Telescope and Gran Telescopio Canarias showed that the contamination of this sample is quite low (≤6%).
[49] In October 2023 an even larger group of 540 planetary-mass object candidates was discovered in the Trapezium Cluster and inner Orion Nebula with JWST.
[4] Observations in 2023 in the Trapezium Cluster with JWST have shown that objects as massive as 0.6 MJ might form on their own, not requiring a steep cut-off mass.
[43] A particular type of globule, called globulettes, are thought to be birthplaces for brown dwarfs and planetary-mass objects.
[53] Some very young star-forming regions, typically younger than 5 million years, sometimes contain isolated planetary-mass objects with infrared excess and signs of accretion.
[54][55][32] Other star-forming regions with iPMOs with disks or accretion are Lupus I,[55] Rho Ophiuchi Cloud Complex,[56] Sigma Orionis cluster,[57] Orion Nebula,[58] Taurus,[56][59] NGC 1333[60] and IC 348.
One Peter Pan disk is the 45 Myr old brown dwarf 2MASS J02265658-5327032 with a mass of about 13.7 MJ, which is close to the planetary-mass regime.
[67] If a stellar or brown dwarf embryo experiences a halted accretion, it could remain low-mass enough to become a planetary-mass object.
This rare encounter can have three outcomes: The iPMO will remain unbound, it could be weakly bound to the star, or it could "kick out" the exoplanet, replacing it.
Simulations have shown that the vast majority of these encounters result in a capture event with the iPMO being weakly bound with a low gravitational binding energy and an elongated highly eccentric orbit.
These orbits are not stable and 90% of these objects gain energy due to planet-planet encounters and are ejected back into interstellar space.
[69] However, in 1998, David J. Stevenson theorized that some planet-sized objects adrift in interstellar space might sustain a thick atmosphere that would not freeze out.
[70] In an Earth-sized object the geothermal energy from residual core radioisotope decay could maintain a surface temperature above the melting point of water,[70] allowing liquid-water oceans to exist.
If they have geodynamo-created protective magnetospheres and sea floor volcanism, hydrothermal vents could provide energy for life.
