Thread by @sraymond_astro

Our new paper was just published in @NatureAstronomy (@nmiretroig, Bouy et al)!

Punchline: we found ~100 free-floating planets in a single star-forming region! This roughly doubles the entire sample of known rogue planets.

A thread

nature.com/articles/s4155…

1/

@nmiretroig and Herve Bouy compiled the census of Upper Scorpius: all the stars, brown dwarfs and rogue planets (>4 Jupiter masses)

They analyzed >80,000 images of Upper Sco from the past 20 years (>100 TB) the Cosmic-DANCE project

project-dance.com

2/

We found about 100 free-floating planets of ~4 to 13 Jupiter masses!

Why “about 100”? Because their true masses depend on the age of the association, which is not well nailed down (3 to 10 Myr).



3/

A younger age for the star-forming region (and therefore the rogue planets)

=> objects are less massive for a given brightness

=> more free-floating *planets* (rather than brown dwarfs, with M>13 MJup).

4/

Are all these rogue planets just tiny failed stars?

Probably not. There are too many free-floating planets when compared with several different initial mass functions.

Models do not predict enough tiny stars.

5/

A significant chunk (up to 90%) of our sample of rogue planets may have formed around stars and been ejected in dynamical instabilities

Dynamical instabilities (aka planet-planet scattering) are nicely illustrated in this animation by Eric Ford



6/

The number of ejected planets depends on 1) the occurrence rate of massive exoplanets around stars, 2) the fraction of systems that go unstable at very early times, and 3) the number of ejected planets per instability

7/

We followed the strategy laid out in Veras & Raymond (2012), using occurrence rates for massive giant exoplanets (from RV, microlensing, direct imaging) and outcomes of planet-planet scattering simulations:

ui.adsabs.harvard.edu/abs/2012MNRAS.…

8/

The action must happen fast! Massive gas giants must form (in groups), go unstable and be ejected within the age of upper Sco (3-10 million years)

(Side note: gas giants going unstable is not good news for rocky planets in those same systems: planetplanet.net/2019/02/06/how…)

9/

Some ejected gas giants hold onto their moons! We can speculate that tidal heating might maintain not-too-frigid temperatures in the interiors of such moons.

planetplanet.net/2017/12/19/exo…

Simulations of moon survival during gas giant instabilities:
ui.adsabs.harvard.edu/abs/2018ApJ...…

10/

For every ejected gas giant there must be a *lot* of ejected Earths. Exactly how many we can’t be sure.

Scaling from microlensing statistics, there should be 10+ super-Earth-mass planets per gas giant.

And Mroz et al found a rogue Earth candidate
ui.adsabs.harvard.edu/abs/2020ApJ...…

11/

Can liquid water or life exist on free-floating planets? We don’t know, of course, but models find that liquid water can persist under a layer of ice or a thick atmosphere of molecular hydrogen.

We can imagine what such life *might* look like: aeon.co/essays/could-w…

12/

The take home message: we now have a big enough population of free-floating planets to constrain star- and planet formation models!

I am grateful to @nmiretroig and Herve Bouy for including me in this really fun and exciting project!

13/

Paper: nature.com/articles/s4155…

ESO: eso.org/public/news/es…
NOIRLab: noirlab.edu/public/news/no…

Nautilus article: nautil.us/blog/we-discov…
Poem: planetplanet.net/2021/12/22/fre…

Amazing animation:

Epilogue:

We now have a sample of >200 free-floating planets vs. just 2 interstellar (aka free-floating) objects.

Your move, @VRubinObs !

Finally -- lead author and astrophysicist extraordinaire @nmiretroig wrote a blog post for @NatureAstronomy about the paper:
astronomycommunity.nature.com/posts/largest-…

Et en francais cette fois-ci (via le @LabAstroBord): astrophy.u-bordeaux.fr/?p=4173