# Solar System Capture of Orphan Planets

NASA recently announced that orphan planets, planets no longer orbiting a star, may be more numerous than the stars in our galaxy.

The Sun's Hill sphere, its gravitational influence within the Milky Way, is ~2.2 ly. So it's possible, however unlikely, that a Jupiter size orphan planet could attain solar system orbit, say with a semi-major axis of one light year and a ~1.6E7 year period. The barycenter of this body and the Sun would, therefore, be ~60.3 AU from the Sun.

Could we detect the Sun's motion around the hypothetical Jupiter barycenter by its gradual anomalous displacement versus distant stars and therefore detect the existence of such a far-off massive planet?

• For the orbital period you mention, I'd imagine the effect on the Sun be very, very small indeed. Also, for a highly elliptical orbit, the magnitude of the effect must also depend on where on the ellipse this 'lonely' planet happened to be at the time; due to its varying angular velocity about the barycenter. Commented May 19, 2011 at 16:10

Dear Michael, yes, you could do it by observing the Sun and stars for those 16 million years or so (the exotic big planet's "year"). Then it would be as visible as the effect of the ordinary Jupiter. Alternatively, you could observe it for 8-16 years only but you would need a million times higher resolution than the resolution for which we may observe the Sun's motion because of Jupiter.

Moreover, this estimate is way too optimistic. The motion of the barycenter would look linear for periods of time much shorter than those 16 million years and to show that the motion is due to an exotic planet, you would surely have to demonstrate some acceleration of the barycenter and maybe even a few higher derivatives to show that it's moving around the curve expected from the extra planets.

In other words, I think it is impossible in practice.

Here is a way measurement of the Sun's anomalous movement could theoretically take place. Assuming a circular orbit (eccentricity = 0) for the distant Jupiter, the Sun's motion around its barycenter would be (2 pi (60.3) AU) / 1.6E7 years or ~3550 km/year. If a detector was placed at the barycenter, 60.3 AU from the Sun, it would find the Sun's annual displacement to be ~0.08 arcseconds. Over 10 years, the ~0.8 arsecond movement would be readily measurable. However, as Luboš pointed out, it's probably impossible to do in practice.

• I think the big issue, is that you are trying the measure the rogue's gravitational attraction on the inner solar system. If this acceleration were constant throughout the inner solar system, it would be indistinguishable from a nearly uniform gravitational field from distant stars. So you need to be able to detect the departure of this baby's gravitational field from uniform, rather than lowest component of the field, and that would be a few orders of magnitude weaker. In your emample the induced anomalous motion of the sun and earth are very similar. Commented May 19, 2011 at 20:54
• No Omega. I'm measuring the Sun's path generated from a hypothetical Jupiter's creation of a barycenter path the Sun follows as the hypothetical orbits the Sun. The measurement takes place at the site of the barycenter. From Earth, the Sun's anomalous movement couldn't be detected per your "the induced anomalous motion of the sun and earth are very similar." Commented May 19, 2011 at 21:53
• @Mike. If we don't know about the planets mass/orbit, we won't know how to identify the barycenter, so I don't think we have detection capability. Commented May 19, 2011 at 22:19
• Omega: I agree that position of the barycenter is important. Commented May 19, 2011 at 23:38
• Although this is an interesting suggestion, I don't see how it ever becomes possible to account for the masses of the billions of objects believed to be orbiting the Sun in the Kuiper belt (at 30 to 50 au), in the scattered disc (50 to 100 au), and in the Oort Cloud (2,000 to 50,000 au) - a region filled with mass, extending out to approximately 1 light year from the Sun. Since the masses and positions of these objects are unknown, it becomes impossible to account for their gravitational effects, which must surely mask any tiny effect from a single object within the scattered disc. Commented May 3, 2021 at 15:41