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Articles »  2021 »  February » Feb 14, 2021

How Newly Discovered Comets Are Named

The number of comets discovered every year is constantly growing, because of the improvements of the available instruments and the execution of sophisticated surveys which aim to automate the discovery of new small bodies in our Solar System. For instance, in 2020 about 73 comets were discovered, while in 2019 the number of discovered comets was about 66. Overall, an average of 6 new comets discovered every month.

Once a comet discovery is confirmed by multiple observations, an provisional name is assigned to the new object. Let's take, as an instance, the famous Comet C/2020 F3 (NEOWISE). What does its name mean?

Comet C/2020 F3 (NEOWISE) captured in July 2020. Copyright © 2021
Comet C/2020 F3 (NEOWISE) captured in July 2020. Copyright © 2021

The first letter of the name identifies the main character of the object. It can take the values C/, P/, I/, D/, A/, X/, where C/ and P/ are by far the most common values. Here is the meaning of the first letter of a comet's name:

  • P/ = The comet is periodic. This means that its orbit is closed (its eccentricity is less than 1), and: it is expected that the comet will return to the inner Solar System in future or; there are already confirmed successful observations of successive returns of the comet.
  • C/ = The comet is non-periodic. This means that its orbit is open (its eccentricity is greater than or equal to 1). The comet is expected to come to the inner Solar System once and to never return, or return after a very long time (more than 200 years)
  • I/ = The comet is an object of interstellar origin. The fact that an object can come from interstellar space can be determined by it's orbital characteristics, like for example an eccentricity significantly greater than 1, denoting an hyperbolic trajectory. The first officially identified interstellar object is 1I/2017 U1 (Oumuamua), whose eccentricity is approximately 1.2.
  • D/ = This denotes a periodic comet that has been observed at least during one passage, but it was successively lost (hence D, from Disappeared).  
  • A/ = This is used to denote objects which were initially identified as possible comets, but further investigations confirmed their minor planet nature. This is typically used when asteroids are discovered on comet-like orbits.
  • X/ = This denotes a comet for which an orbit could not be determined (for instance historical comets for which precise observations are not available)

The second part of the name, the number after the first letter, is the year when the comet was discovered. This, for instance, means that C/2020 F3 (NEOWISE) was discovered in 2020.

Than we have a combination of a character and a number. The character identifies the half month from the beginning of the year. A means the first half of January; B the second half of January; C the first half of February; D the second half of February; and so on. The number after the character identifies the progressive number of the discovery in that time period. F3, for instance, means the third discovered comet in the second half of March.

Finally the comet's name is completed by a name, enclosed between brackets. This is used to attribute the discovery to the author, or authors. It can be the surnames of the discoverers, or the name of the sky survey that discovered it.

Anatomy of a comet's provisional name
Anatomy of a comet's provisional name

(Published on February 14, 2021)