Saturday, August 7, 2021

How exoplanets are named?

Whenever astronomers discover any new celestial body then it is named according to some defined and specified rules. These rules or nomenclature methods are different for different bodies. Whether its asteroids or comets or any exoplanet, everything that is newly discovered has some specified rules to follow before naming them. 

In this article we are going to see how a newly discovered exoplanet is named. So let’s start.

Traditional way of nomenclature

Earlier when exoplanets were started to discover there were no pre-defined rule to name them so these exoplanets were given some specified names. For example the exoplanet orbiting around 51 Peg was named Bellerophon. Similarly Methuselah was the name given to the planet orbiting the pulsar white dwarf binary PSR B1620-26, and so on.

But there was a problem in naming exoplanets like this which is as large numbers of exoplanets started to be found, this method was found too complex as every time a new exoplanet is discovered some new and unique name was required so it was replaced by an adaptation of the system used for naming binary stars.

Using alphabet letters for nomenclature

A commonly used convention for naming exoplanet is to label exoplanets by the lower case letters b, c, d, …. following the name of the star (which is designated by an ‘a’ or by no extra label) in the order of their discovery or, for two or more exoplanets discovered simultaneously, in the order of their distance from the star.

For example, 55 Cnc is a binary star system about 40 light years away from us with a yellow dwarf primary and a red dwarf secondary. Five exoplanets have now been detected orbiting the yellow dwarf.

The nomenclature for this system is thus 55 Cnc A (yellow dwarf star) 55 Cnc B (red dwarf star) 55 Cnc Ab, 55 Cnc Ac, 55 Cnc Ad, 55 Cnc Ae and 55 Cnc Af. You can also see these exoplanets labelled as 55 Cnc b, 55 Cnc c, 55 Cnc d, 55 Cnc e and 55 Cnc f, which is the same thing just another way of writing. (Read – Types of exoplanets)

Nomenclature using the device name used in its discovery

This method of nomenclature uses the same letters but combined with the instrument that made the discovery and a running number giving the order of that discovery. Thus we have Kepler-4 b which is the first exoplanet discovered by the Kepler spacecraft (Kepler-1 b to Kepler-3 b were already known exoplanets). 

Similarly exoplanet discovered from the Optical Gravitational Lensing Experiment (OGLE) is named like  OGLE-TR-10 b, if it is discovered from the Wide Angle Search for Planets (WASP) then it is named like WASP-6 b and if discovered from the Convection, Rotation and planetary Transit spacecraft (CoRoT) then naming will look something like this CoRoT-7 c.

OGLE can detect exoplanets via gravitational microlensing events and via the MOA (Microlensing Observations in Astrophysics) also. So names for planets discovered using these methods will be like OGLE-2005-071 L b and MOA-2007-BLG-400-L b.

The format for naming exoplanet is in this way : Search name – Year of the microlensing event – sequential number of the event – L for a lensing-based exoplanet discovery (TR for a transit discovery). If the term ‘BLG’ is included then it denotes that the star is in the Milky Way’s central galactic bulge.

How the first discovered exoplanets were named?

As it was mentioned in the starting of the article that no convention for naming exoplanets existed earlier and when, in 1992, the first two exoplanets were found orbiting the pulsar PSR 1257+12, they were labelled as PSR 1257+12 B and PSR 1257+12 C. 

Two years later, a third exoplanet discovered closer in towards the pulsar was called PSR 1257+12 A and in 2002 a fourth member of the system became PSR 1257+12 D. These names are still in use although the labels PSR 1257+12 a, PSR 1257+12 b, etc. are also found in many books and scientific articles. 

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