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I was perusing the exoplanet database and looking for some of the more extreme orbits, when I happened upon HD 20782 B:

http://exoplanets.org/detail/HD_20782_b

Which is also listed (in two sentences) on wikipedia, so there's not a whole lot of information. However, the first thing I noticed was that it was almost at escape velocity!

enter image description here

I guess not much is known, but my overall question is:

Would an exoplanet on an escape trajectory offer us any opportunities for research that have yet to be performed? What would those opportunities be? The answer can be a justified no (E.G. once the planet leaves the host star, we can't observe the doppler shift anymore).


Imaginary Bonus Points: have there ever been any observed as escaping/becoming rogue planets?

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The problem is that planet ejection is something that takes place over a very short timescale compared to the ages of the systems in question, so we're unlikely to see it actually take place. The main prospect of detection would be imaging of the ejected planets, for which you'd want a young giant planet as these retain enough heat from their formation to be sufficiently luminous in the infrared for detection even when not illuminated by a nearby star.

There has actually been a claim for the detection of a (proto)planet in the process of being ejected from a binary star system: the case of TMR-1C as reported by Terebey et al. (1998) "A Candidate Protoplanet in the Taurus Star-forming Region". Unfortunately the nature of the object is not confirmed, see Terebey et al. (2000) "The spectrum of TMR-1C is consistent with a background star". There has been some back-and-forth on this: Riaz & Martín (2011) "Large-amplitude photometric variability of the candidate protoplanet TMR-1C " reject the background star hypothesis, while Riaz et al. (2013) "A near-infrared variability campaign of TMR-1: New light on the nature of the candidate protoplanet TMR-1C" regard it as a strong candidate for being a young stellar object (YSO).

Another relevant case might be the circumbinary planet HD 106906 (AB) b, which may have been saved from ejection by a stellar flyby, see De Rosa & Kalas (2019) "A Near-coplanar Stellar Flyby of the Planet Host Star HD 106906".

Ejected planets are of interest due to their contribution to the population of low-mass objects in star clusters and the field (this population may well contain both ejected planets and low-mass brown dwarfs), and the impacts on the orbital architecture of the surviving planets after episodes of planet-planet scattering. One example of the latter I found is Carrera et al. (2019) "Planet-planet scattering as the source of the highest eccentricity exoplanets", there are many other papers on this topic.

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Source: https://exoplanets.nasa.gov/5-ways-to-find-a-planet/

Currently, we have five known ways of detecting exoplanets. Your question asked about observing planets that are escaping stellar systems. These are detecting shifts in stellar velocity, detecting shifts in stellar position, searching for transits, direct imaging, and detecting the gravitational lensing of light.

A planet that's being ejected from the orbit of a star would have increasingly small gravitational effects on its parent star as it flies further away through space. It would also no longer have a regular orbit and transits and would be poorly lit in the interstellar medium, making our detection efforts largely in vain. So I would suggest that the data we could glean from a planet going rogue is about the same data we can glean from any regular exoplanet.

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    $\begingroup$ The last sentence is unjustified. "So I would suggest that the data we could glean from a planet going rogue is about the same data we can glean from any regular exoplanet." If a planet is light years away from any star, then probably there would be no opportunities for research whatsoever because it would not be in any way detectable. Unless of course it got close enough to our solar system that it could cause some measurable gravitational perturbations or start interacting with light from our Sun. $\endgroup$ – uhoh May 23 '19 at 7:18
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    $\begingroup$ "A planet that's being ejected from the orbit of a star would have increasingly small gravitational effects on its parent star as it flies further away through space. " Which is of course a very unlikely event. In orbital mechanics, you're bound or you're unbound. Interaction with a third body can destabilize orbits, but that doesn't necessarily change an ellipse into a hyperbola. $\endgroup$ – AtmosphericPrisonEscape May 23 '19 at 10:32
  • $\begingroup$ uhoh: I assumed the question asked about a planet near the star on an escape trajectory. AtmosphericPrisonEscape: Planets are detected and observed by their gravitational effects on their parent star. (Doppler-shift detection works this way.) Thus, as a planet flies farther, it would have less gravitational pull on its parent star and less opportunity to be detected. $\endgroup$ – Cloudy7 May 23 '19 at 19:36

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