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Recently, planets outside our galaxy have been discovered.

How would be the convention to naming these planets? As far as I know, it seems that there is no standard for naming exoplanets even in our galaxy.

But if I'm publishing a scientific paper and I wish to name some of these recently discovered planets, what would be some guildelines on doing so?

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  • $\begingroup$ You look up the discovery paper. They will have catalogue names. The convention for a planet around a Star called "Star123" would be to call it "Star123b". Not sure about those Quasar-planets, I have yet to read the original paper. As we cannot resolve individual stars or assign them to any host star in those high-redshift galaxies, they'll probably end up having simple numberings. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Feb 5, 2018 at 0:46
  • $\begingroup$ You are correct that there is no single naming convention for unbound exoplanets in our galaxy (@AtmosphericPrisonEscape describes the convention for bound exoplanets). As StephenG pointed out in his answer, individual planets have not been identified, so they can't really be named individually anyway. That said, you could refer to them as a group "unbound planets of RX J1131-1231" (a mouthful). I imagine, though, that many of these might not be unbound planets at all, but planetary mass brown dwarfs. $\endgroup$
    – called2voyage ♦
    Commented Feb 5, 2018 at 13:27
  • $\begingroup$ (The authors do say in their paper that they are really just constraining the fraction of planetary mass objects. "Extragalactic planets" just sounds more exciting.) $\endgroup$
    – called2voyage ♦
    Commented Feb 5, 2018 at 13:29

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The original paper ( and a version on arXiv ) which this relates to does not seem to identify any actual planets. It's clear from the abstract that it is simply suggesting there is evidence that what they describe as unbound planets seem to exist as an inference of the spectral data they have gathered.

They are not identifying any individual planets at all. They have not actually discovered any planets and there is nothing to name.

This is the abstract :

Previously, planets have been detected only in the Milky Way galaxy. 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.

What they're saying is that, if you accept the hypothesis of their theory and data, there must exist a minimum of about 2000 objects of planet mass per star. There's no way to identify any individual object and no means to verify their claims that I know of.

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It's a little bit like naming people off the shadows they cast walking past a light-source. It's hard to tell which shadow belongs to what person. A guy who walks by twice might get two names, two people who walk by at the same time might get one name between them. The lack of specific identification is the difference between direct observation and a survey. It's not even all that easy to tell if a person is tall or just closer to the light source - though some rough estimates can be made, specifics are difficult.

We don't even name the planets in our own galaxy that are observed in microlensing events. We could name the events, I suppose (and maybe they do). But there's no way to identify and track individual planets from microlensing events.

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