It doesn’t seem to be part of the Proxima Centauri system, it must be closer and correspondingly smaller. With just 2 observations it isn’t possible to determine the object’s orbit, so we can only guess at its distance and size. One possibility (and the one I think most likely) is that it’s an extreme trans-Neptunian object ~100 astronomical units away from the Sun, which is further than Sedna at 86 AU. This would make it the most distant known object in the solar system, but likely smaller than Pluto.

Another possibility (which seems more likely to the object’s discoverers) is that it is ~300 AU away and ~1.5 times the size of Earth, making it the first “super-Earth” found in our solar system. Observations of trans-Neptunian objects have led to some speculation that 1 or 2 super-Earths could lurk in the outer solar system, so it’s not out of the question. There’s reason to be cautious of this idea, however, because of its location. Proxima Centauri is ~42 degrees away from the ecliptic. Most large solar system lay within a few degrees of the ecliptic, and even Sedna’s orbit is only inclined ~12 degrees from it.

A third possibility is that the object is a cool brown dwarf ~20k AU away. Such an object should also be visible in the infrared, so there would still be the question as to why it wasn’t discovered by earlier infrared sky surveys. Its proximity to Proxima Centauri would seem to make such an object easy to find.

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