[peeters]

One thing to keep in mind is that there is one thing that happens with exoplanets that can never happen with Pluto: the planet can pass between its star and us (i.e. eclipse the star). This gives a lot of information about the planet's composition, because light from the star will scatter/change color according to the planet's composition. See this image: http://seagerexoplanets.mit.edu/images/transitschematic.gif.

Now maybe you can do that with Pluto using some far-off star for reference, not sure. Also, I'm not sure if in this specific example the exoplanet made a transit of its star, or if it was detected by other means (usually by measuring its gravitational affect on the star, i.e. its star "wobbles").

[verytrivial]

Indeed this is exactly what New Horizons did very recently. After shooting past Pluto, the probe spun around to view it with the sun behind it. http://pluto.jhuapl.edu/Multimedia/Science-Photos/image.php?...

[privong]

> Now maybe you can do that with Pluto using some far-off star for reference, not sure.

It has been done for Pluto, first in 1988, with an occultation of a background star[0].

[0] http://www.space.com/29885-pluto-atmosphere-to-be-revealed-b...

[goodcanadian]

And most recently with SOFIA at the end of June:

https://sofia.usra.edu/News/news_2015/06_29_15/index.html

This will help tie in past and future occultation observations to the New Horizons data.

[dr_zoidberg]

As I understand, star transits by Pluto helped to better estimate its size before NH. But as you said, it's complicated to get a full spectrographic analisys of its atmosphere this way.

Kepler is a mission that mostly works with photometric measurements of planet transits in front of their stars.

[Sharlin]

And indeed the fact that the observed transits were "fuzzy" told us that Pluto has a (very tenuous) atmosphere. Indeed, Pluto's diameter had considerable uncertainty before New Horizons arrived due to this (unlike Charon's whose transits were always tack sharp.)