The Strange Orbits of “Tatooine” Planet Disks

by | Mar 23, 2020 | Exoplanets, Planets, Stars | 0 comments

The Strange Orbits of “Tatooine” Planet Disks
Two examples of aligned and misaligned protoplanetary disks around binary stars (circumbinary disks), observed with ALMA. Binary star orbits are added for clarity. Left: in star system HD 98800 B, the disk is misaligned with inner binary stars. The stars are orbiting each other (in this view, towards and away from us) in 315 days. Right: in star system AK Scotii, the disk is in line with the orbit of its binary stars. The stars are orbiting each other in 13.6 days. CREDIT: ALMA (ESO/NAOJ/NRAO), I. Czekala and G. Kennedy; NRAO/AUI/NSF, S. Dagnello

And in one final bit of planetary news, astronomers have looked at a dozen planetary systems that have formed around binary stars. Called “Tatooine” systems, these systems exhibit some neat relationships between the stars’ orbits and the planets’ orbits. The illustration that accompanied this story may mislead folks. In general, the planets orbit in circles, but those circles look like ovals when you rotate the planet’s orbit relative to the star’s orbit. And this lack of alignment is exactly what happens in systems where the stars have longer orbits. This means that in systems where the planets are orbiting in and out of the sky – toward and away from us – we will observe planets going in a polite circle on the sky when the orbit is long [LEFT IMAGE]. This was seen for HD 9880 B and it’s 315 day orbit. In a system that has the stars still orbiting toward and away from us but in a shorter orbit, the planets will also orbit toward and away from us, nicely aligned [RIGHT IMAGE], as is seen in AK Scotii, where the stars orbit in 13.6 days.


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