In the Late-2020s a microlensing survey could tell if Rogue planets are more common than planets around Stars

The Wide Field Infrared Survey Telescope (WFIRST) is a proposed infrared space observatory which was selected by National Research Council committee as the top priority for the next decade of astronomy. The WFIRST space telescope could be in space by 2024 if it is started in 2017.

Estimates suggested that every planetary system in the galaxy booted at least one planet into interstellar space. With billions of planetary systems in the Milky Way, there may be billions, maybe even hundreds of billions, of rogue planets in the galaxy, says planetary scientist Sara Seager of MIT.

“A census of rogues,” Liu says, “is the only way we are going to fully understand the extent of what’s out there in the Milky Way.”

Two traits distinguish a star from a brown dwarf and to an extent, from a planet: mass and the presence or absence of nuclear fusion. Stars, even small ones, are at least 80 times the mass of Jupiter, which at 318 times the mass of Earth is the most massive planet in the solar system — and is often used by astronomers to gauge the size of other gaseous objects. According to theoretical calculations about how stars work, objects must be 80 Jupiter masses or more to fuse hydrogen nuclei (protons) into helium. This process liberates energy, which is how stars burn bright, speckling the night sky.

Brown dwarfs are smaller, anywhere between 13 and 80 Jupiter masses. They are not dense enough to fuse hydrogen. But they may have been big and hot enough to fuse deuterium nuclei (a proton plus a neutron) with protons or other nuclei, which means they once generated energy but no longer do.

Any sphere less than about 13 Jupiter masses is not large or dense enough to fuse any kind of atomic nuclei. As a result, some astronomers define orbs with less than roughly 13 Jupiter masses — even untethered ones — as planets.

One study suggests there could be 100,000 rogue planets for every star in the Milky way.

Astronomy and Astrophysics – CFBDSIR2149-0403: a 4–7 Jupiter-mass free-floating planet in the young moving group AB Doradus?

MASS MATTERS Small stars, brown dwarfs and rogue planets can be similar in diameter but have different masses. Mass is one characteristic used to distinguish the objects. However, for classification purposes, astronomers may need to look beyond mass to consider how an orb formed and what elements it’s made of.

Abstract -Astronomy and Astrophysics – CFBDSIR2149-0403: a 4–7 Jupiter-mass free-floating planet in the young moving group AB Doradus?

Using the CFBDSIR wide field survey for brown dwarfs, we identified CFBDSIRJ214947.2-040308.9, a late T dwarf with an atypically red J − KS colour. We obtained an X-Shooter spectra, with signal detectable from 0.8 μm to 2.3 μm, which confirmed a T7 spectral type with an enhanced Ks-band flux indicative of a potentially low-gravity, young object. The comparison of our near infrared spectrum with atmosphere models for solar metallicity shows that CFBDSIRJ214947.2-040308.9 is probably a 650−750 K, log g = 3.75−4.0 substellar object. Using evolution models, this translates into a planetary mass object with an age in the 20−200 Myr range. An independent Bayesian analysis from proper motion measurements results in a 87% probability that this free-floating planet is a member of the 50−120 Myr-old AB Doradus moving group, which strengthens the spectroscopic diagnosis of youth. By combining our atmospheric characterisation with the age and metallicity constraints arising from the probable membership to the AB Doradus moving group, we find that CFBDSIRJ214947.2-040308.9 is probably a 4−7 Jupiter mass, free-floating planet with an effective temperature of ~700 K and a log g of ~4.0, typical of the late T-type exoplanets that are targeted by direct imaging. We stress that this object could be used as a benchmark for understanding the physics of the similar T-type exoplanets that will be discovered by the upcoming high-contrast imagers.

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