Title: A nomenclature for individual exocomets
Authors: Alain Lecavelier des Etangs, Paul Strøm, Darryl Z. Seligman
First Author’s Institution: Institut d’astrophysique de Paris (CNRS, Sorbonne Université)
Status: arXiv preprint (open)
The problem: exocomets are having their “we need a filing system” era
Exocomets used to live in the realm of hints– mysterious dips in light curves, transient absorption features, “something weird is happening around this star.” But the authors of today’s paper argue that we’ve crossed a threshold: we’re now studying individual events in enough detail that we need to be able to reliably say which exocomet we’re talking about, the same way we do for exoplanets and Solar System minor bodies.
And right now… the naming situation is a little chaotic. One object/event can accumulate multiple labels depending on the team, the instrument, the catalog name of the star, or the style of the paper. The whole point of a nomenclature is to avoid exactly that problem: one object → one unambiguous designation.
So today’s paper is basically a plea (and a proposal): let’s give exocomets a consistent naming convention before the literature becomes a scavenger hunt.
Comet Nomenclature 101 (Solar System edition)
Before we hop out of the Solar System, it helps to see how comet names work in our turf.
Step 1: the leading letter tells you what kind of comet (or “comet-ish thing”) it is
The International Astronomical Union’s coded system uses a first letter that flags the category:
- P/ = periodic comet (orbital period < 200 years, or confirmed to have returned)
- C/ = non-periodic / long-period comet (typically > 200 years, seen once)
- X/ = comet with no reliable orbit (often from historical records)
- D/ = periodic comet that’s lost / disrupted / gone
- A/ = object first tagged as a comet, later reclassified as a minor planet
- I/ = interstellar object (category introduced in 2017)
Concrete examples:
- 1P/Halley is the classic periodic comet.
- 67P/Churyumov–Gerasimenko is another numbered periodic comet.
- C/2020 F3 (NEOWISE) is a non-periodic/long-period comet.
- D/1993 F2 (Shoemaker–Levy 9) carries the “D/” because it broke apart and is effectively “gone” as a single comet now (famously after meeting Jupiter).
- 1I/2017 U1 (‘Oumuamua) shows the “I/” interstellar category in action.
Step 2: the discovery timestamp code
For a “C/” comet, the rest of the code usually encodes:
- the year of discovery,
- a half-month letter (A = first half of January, B = second half of January, C = first half of February…),
- and a number giving the order of discovery in that half-month.
Some examples:
- C/1995 O1 means the first comet discovered in the second half of July 1995 (Hale–Bopp).
- C/2020 F3 means the third comet discovered in the second half of March 2020 (NEOWISE).
Step 3: bonus “numbering” for periodic comets
Once a comet’s periodicity is confirmed, it can get a number in front of the P (e.g., 1P, 67P), reflecting its order in the list of recognized periodic comets.
Okay, now… exocomets. Why is naming harder?
Exocomets aren’t usually seen as a tidy nucleus with a pretty tail. We detect them indirectly: through variable absorption features (famously around β Pictoris) or asymmetric transit dips caused by dust tails. The awkward part is that you can get many events around the same star, sometimes even multiple “comets” in the same dataset window—so a naming scheme needs to be simple but also not ambiguous.
The authors highlight another subtle choice: if your label starts with the host star name, you’re implicitly defining an exocomet as something bound to a star—which means the scheme intentionally excludes free-floating / interstellar comets from the “exocomet” category.
The proposal: a similar system of tagging
The authors of today’s paper recommend this format: StarName C yyyymmdd [optional letter]
Where:
- StarName anchors the event to a specific planetary system (like exoplanet names do).
- They recommend prioritizing short, widely recognized stellar designations (Bayer → Flamsteed → HD → major survey IDs) to reduce alias chaos.
- The C is a deliberate nod to Solar System comet nomenclature: it flags “cometary nature.”
- They considered using E (for “exocomet”), but rejected it: “evaporating” is misleading (sublimation is expected), and “E” could apply to lots of extrasolar objects (exo-asteroids, exo-moons…).
- The choice of C was discussed at a workshop in July 2024 by the International Space Science Institute.
- yyyymmdd is the date of the discovery or characterizing observation.
- They explicitly don’t use the Solar System half-month code (like “F3”), because exocomet systems can produce many events within the same half-month (β Pic being the poster child).
- They also like that these dates mirror the naming logic of other transient events (gamma-ray bursts), and help connect repeat observations later.
- Optional a/b/c… if multiple exocomets are seen on the same day around the same star.
- Discoverers choose the ordering; if it ever gets messy with independent teams, they suggest adding fractional-day precision.
What they don’t include (yet): periodicity, method tags, physical properties
This part feels like the authors are pre-empting feature creep.
- You could imagine encoding periodicity like Solar System P/ comets (they even mention 1P/Halley and 67P as the analogy), but they argue periodicity is not accessible for most exocomet detections, so they leave it out.
- You could encode the detection method (spectroscopy vs photometry vs imaging) or even radial velocity to distinguish multiple comets in one spectrum, but they consider that to be too complex unless/until it becomes necessary.
Examples: does this naming scheme actually look usable?
The authors give worked examples to show how the exocomet names would be read in practice. A few from the paper:
- β Pic C19841113 (pointing to an early, clearly identifiable β Pic event).
- β Pic C19971206 (an Hubble Space Telescope-observed transit signature they discuss as a specific event).
- HD 172555 C20110611.
- KIC 846285 C20090805 (Tabby’s Star / Kepler photometry case; possibly a string of exocomets).
Why this matters
This is one of those “boring on purpose” papers that saves everyone time later.
A simple standard:
- reduces ambiguity across studies,
- makes it easier to cross-match events across instruments/archives,
- and avoids the slow drift toward “everyone uses their own label and we all pretend it’s fine.”
And if exocomet science keeps trending toward individual characterization—composition constraints, tail morphology, event rates, family groupings—then having a clean way to refer to “that specific event” becomes necessary infrastructure, not just aesthetics.
Edited by: Anavi Uppal
Featured Image Credit: ESO/L. Calçada
