After the first round of popular news items about the "first extragalactic exoplanet" discovery, CNET's Did astronomers find the first planet outside of the Milky Way? It's complicated points out:
While many news sources have championed the detection as the "first planet discovered outside of the Milky Way," there's no way of confirming the find.
An event several hours long that is not expected to recur for many decades is pretty much unconformable in foreseeable future.
The dip in X-ray brightness is apparent on this graph, just prior to 45 hours -- but was it caused by a planet? NASA/CXC/SAO/R. DiStefano, et al.
Later in the article:
Pope1 is less convinced. "Personally, I wouldn't bet that this is a planet," he says. "In my view this is probably a stellar companion or something exotic happening in the disk."
Trust the process
This isn't the first time NASA's Chandra observatory has been swept up in a potential "extroplanet" find. Studying how radiation from distant stars is "bent" by gravity, a technique known as microlensing, astronomers at the University of Oklahoma believed they detected thousands of extragalactic planets back in 2018. Earlier studies have claimed to find evidence of extragalactic planets in the Andromeda galaxy.
Other astronomers were skeptical about these detections, too. The same skepticism has played out in the case of M51-1. And, importantly, that's perfectly normal.
1"Benjamin Pope, an astrophysicist studying exoplanets at the University of Queensland in Australia."
Prior to 2018 definitely rules out the TESS survey, as does the concentration of observations within the Andromeda galaxy, only a few degrees long. From only this one sentence it's hard to tell what methods were used to search for exoplanets.
Question: Which studies prior to 2018 "claimed to find evidence of extragalactic planets in the Andromeda galaxy"? Which instrument was used?