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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.

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."

To have thousands of potential objects suggests a survey, and exoplanet detection via gravitational microlensing suggests photometry. 2018 suggests early data from TESS, the Transiting Exoplanet Survey Satellite, but it could also have been a different instrument.

Question: Which University of Oklahoma research purported to "detect thousands of extragalactic planets back in 2018"? Which instrument was used?

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The press release associated with the 2018 study is here; the actual article is Dai & Guerras (2018). From the abstract:

Here, we show that quasar microlensing provides a means to probe extragalactic planets in the lens galaxy, by studying the microlensing properties of emission close to the event horizon of the supermassive black hole of the background quasar, using the current generation telescopes. We show that a population of unbound planets between stars with masses ranging from Moon to Jupiter masses is needed to explain the frequent Fe Kα line energy shifts observed in the gravitationally lensed quasar RXJ 1131-1231 at a lens redshift of z = 0.295 or 3.8 billion lt-yr away. We constrain the planet mass-fraction to be larger than 0.0001 of the halo mass, which is equivalent to 2000 objects ranging from Moon to Jupiter mass per main-sequence star.

They are using a distant AGN (RXJ 1131–1231), at a redshift of 0.658, which forms a quadruple lens due to a foreground galaxy (at a redshift of 0.295). The argument is that if you focus on the X-ray emission, you are getting light from a very small spatial region around the AGN's SMBH. If this is comparable in size to the Einstein-ring radius of an object (star or planet) in the lensing galaxy, then you can get significant amplification of the signal while the object is passing very near the line of sight to the AGN. There's an important additional effect, where emission from different regions of the AGN accretion disk are amplified, and so you get an emphasis on different Doppler shifts (e.g., if the planet's lensing amplifies part of the accretion disk rotating towards us, then you might measure an X-ray emission line with a slightly blueshift relative to the AGN's main redshift).

So, using observations of all four lens images with the Chandra X-ray Observatory at 38 different epochs over a period of about a decade, they observed a number of different redshifts and blueshifts of the iron K$\alpha$ X-ray emission line. (This was presented in previous papers.) They then attempt (in the 2018 paper) to model the overall number and distribution of red- and blue-shifted microlensing events. (By "event" I mean the observation of an emission-line redshift or blueshift at a particular epoch; they're not tracking microlensing light curves.)

Their argument is that they can't reproduce the data using just an assumed population of stars (and brown dwarfs); they need a population of additional, lower-mass objects. These are postulated to be free-floating ("rogue") planets/planetary-mass-objects, because the effect of a planet orbiting a star would be lost in the larger effect of the star itself.

It's important to note that they aren't quite detecting individual (rogue) planets; they're deducing a population of such objects from the statistical effect.

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