What are comets called?

Comets are not just named. Since 1995, the International Astronomical Union has ensured that the nomenclature is respected. How are the names of comets chosen?

Like the names of exoplanets, those of comets are not chosen at random. Over 4,500 of these objects have been discovered, including at least sixty this year 2021. To find your way around, we must therefore name each of these frozen relics of the formation of the solar system. Some are famous, like 1P / Halley (Halley’s comet), others a little less, like C / 2018 C2, C / 2016 E2 (Kowalski) or C / 2021 E3 (ZTF).

How did we choose these names to designate comets? As explain it The Institute of Celestial Mechanics and Ephemeris Computation (IMCCE) of the Paris Observatory, the tradition first consisted of ” give comets the name (s) of their discoverer (s) », With the possibility of indicating up to three names if the object was discovered by several people. However, we realized the ambiguities of this method, because discoverers had spotted several comets. We were thus able to end up with two Huyakutake comets, three P / Hartley comets and about twenty Bradfield comets! A nomenclature had to be imagined and required several adjustments over time.

The current nomenclature is used since 1995 : the International Astronomical Union (IAU) is responsible. The Center of the minor planets (MPC, which depends on the IAU) deals with the discovery and confirmation of new comets. The IAU ensures that the object is correctly named while respecting the nomenclature.

A letter, a number, a letter, a number

The name of a comet always begins with a letter, which refers to the type of comet. The prefix must be one of the following:

  • « P/ »For a periodic comet (which takes less than 200 years to circle the Sun),
  • « C/ »For a non-periodic comet (which takes longer to revolve around the Sun) – like C / 2020 F3 (NEOWISE),
  • « X/ »For a comet whose orbit cannot be calculated correctly,
  • « D/ “For a disappeared periodic comet,
  • « I/ For an interstellar object, whether it is a comet – like 2I / Borissov – or an asteroid.

The name is then formed with:

  • L’year the discovery of the comet,
  • A capital letter which makes it possible to identify the fortnight of the month in which the comet was observed (“A” for the first fifteen days of January, “B” for the following fifteen days, etc.),
  • a Name which corresponds to the order of discovery of the object during this fortnight.
Your turn: this is a comet that you have just discovered. What would you name her? // Source: Nino Barbey for Numerama

To respect tradition, we can finally add the name of the discoverer in parentheses. For example, “C / 1996 B2 (Hyakutake)” is obtained to name the long-period comet discovered by Yuji Hyakutake in the second half of January 1996.

Obviously, it would be too easy to stop there. Thus, for short period comets, of which more than one return has been observed, one adds before the prefix “P /” a number and one abandons the other numbering system – which gives “1P / Halley”, or “46P / Wirtanen”. There are exceptions, with comets taking the name of their discoverer or the instrument with which they were discovered. There are also asteroids whose cometary activity has been discovered, reclassified as comet by adding the prefix “P /” in front of the designation as asteroid.

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