In 2019 the Nobel Prize in Physics has been awarded to Michel Mayor and Didier Queloz for the discovery of the first exoplanet around a Sun-like star. I would like to point out that the planet has not been discovered in 2019 or any year near that. 51 Pegasi b, was the first exoplanet ever found around a Sun-type star, is discovery was announced on 6 October 1995 by Mayor and Queloz, who detected it using the ELODIE spectrograph at the Observatoire de Haute-Provence in France. This discovery revolutionised astronomy, initiating an entirely new field and new instruments focused on finding and characterising exoplanets.
Figure 1: Didier Queloz and Michel Mayor at La Silla
Introduction
Exoplanets or extrasolar planets, are planets outside the Solar System. From 1917 until 8 February 2021, has been discovered a total of 4,352 exoplanets in 3,257 systems, with 722 systems hosting more than one planet.
You might ask, how do we know that planets are there? How he can detect relativity small objects that are very far away? Well, we do it in many ways, but most of the planets were found using one of the following methods. Transit photometry or Doppler spectroscopy. However these methods suffer from a clear observational bias favouring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone. As have been told before multiple systems have more than one planet orbiting the star. Studies point to the fact that 1 in 5 Sun-like stars have an "Earth-sized" planet, orbiting the star habitable zone. Knowing this and looking just to the Milky Way with all 200 billion stars (more or less), we are looking (hypothetically) at 11 billion potentially habitable Earth-sized planets, if we put the red dwarfs in the equation the number rises to 40 billion.
Figure 2: Discovered exoplanets each year with discovery methods as of 5 March 2020
Draugr (also known as PSR B1257+12 A or PSR B1257+12 b) and HR 2562 b, are two very distinct planets. Draugr is the least massive planet, known so far, which is about twice the mass of the Moon. On the other side, we have HR 2562 b, which is about 30 times the mass of Jupiter. Almost all the planets detected so far are within the Milky Way. The nearest exoplanets are located 4.2 light-years from Earth, orbiting Proxima Centauri, the closest star to the Sun.
With the discovery of exoplanets that fulfil some characteristics, the search for extra-terrestrial life has intensified. There is a special interest in planets that orbit in a star's habitable zone, where is possible for the existence of liquid water, a prerequisite for life on Earth, to exist on the surface. They also consider a bunch of other factors to determine the planet's suitability for hosting life.
Exoplanets also integrate planets that do not orbit any star, such planets are called rogue planets. Such objects are considered a separate category of the planet, especially if they are gas giants, which are often counted as a sub-brown dwarf. In our Milky Way, alone, may exist billions or more rogue planets.
Definition
So, you may be asking, what exactly defines an exoplanet? So, an exoplanet is defined by the following criteria:
- Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral < 2/(25+√621) are "planets" (no matter how they formed).
- The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
However different organizations use different criteria. While the Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, claiming that there is no special feature around 13 MJup in the observed mass spectrum, reinforces the choice to forget this mass limit. In 2016 this limit was increased to 60 Jupiter masses based on a study of the relation between the mass and the density of the object. The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The13 Jupiter-mass distinction by the IUA Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity." The Nasa Exoplanet Archive includes objects with a mass equal to or less than 30 Jupiter masses. Another used to separate planets from brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core is dominated by coulomb pressure or electron degeneracy pressure with the dividing line at around 5 Jupiter masses.
Nomenclature
Sure you have asked why the scientist give so strange names to the planets they find. Well, the reason for that, is the planet name results from an extension of the system used for designating multiple-star systems as adopted by the IUA. For exoplanets orbiting a single star, the name given to the planet is formed by taking the designated or proper name of its parent star, and adding a lowercase letter. The letter is given in order of each planet's discovery around the star, the first planet discovered in a system is designated "b" (the letter "a" refers to the parent star) and later planets are given subsequent letters. It could happen that multiple planets are discovered at once orbiting the same star, in this case, the closest planet to the star gets the letter "b", the next gets the letter "c" and it follows like this in order of orbital size.
Detection
For a long time philosophers, scientists and science fiction writers believed in the existence of exoplanets, however, they had no way to prove their existence. The first known man to think of the existence of exoplanets (at least publicly known) was Giordano Bruno, in the sixteenth century. Later, in the eighteenth century, the same possibility was mentioned by Isaac Newton. However, with speculations scientific progress goes nowhere, the first confirmation of the detection, of an exoplanet, came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. Later in 1995, a giant planet was found orbiting a main-sequence star, 51 Pegasi (name of the star). Exoplanets can be imaged directly by telescopes, and some have been, however, almost every exoplanet discovered has been through indirect methods, such as the transit method and the radial-velocity method.
Doppler Spectroscopy
Doppler spectroscopy or the radial-velocity method, is an indirect method used to find extrasolar planets and brown dwarfs. It does it by measurements of the radial velocity via observation of Doppler shifts in the spectrum of the planet´s parent star. Until February of 2020 880 extrasolar planets were found using this method (about 21% of the total). Using this method scientists can determine the minimum mass of the planet from the changes in the star's radial velocity. The first non-transiting planet to have its mass found by this method was Tau Boötis b in 2012 when carbon monoxide was detected in the infra-red part of the spectrum.
Figure 3: Diagram showing how a smaller object (such as an extrasolar planet) orbiting a larger object (such as a star) could produce changes in position and velocity of the latter as they orbit their common centre of mass (red cross).
Transit Photometry
With the radial-velocity method, he can obtain information about the planet's mass, and the photometric method provides us information about the planet's radius. When a planet crosses or transits in front of its parent star's disk, the observed visual brightness of the star drops by a small amount. How much it drops? Well, that depends on the size of the planet relative to the star. For example, an Earth-size planet's transiting a Sun-like star produces a dimming of 0.008%.
This method has two major disadvantages. First, the planet's transit is visible only if the planet's orbit happens to be perfectly aligned from an astronomer's point of view. The probability of that happening is low, however, pointing the telescopes to the night sky and observing large areas of the sky, containing thousands or even hundreds of thousands of stars at once, transit surveys can find more extrasolar planets than the radial-velocity method. Second, this method has a high rate of false detections.
2019 Physics Nobel Prize
As I told you before, part of the 2019 Physics Nobel Prize was given to Michel Mayor and Didier Queloz for the discovery of 51 Pegasi b. Also known as Dimidium, 51 Pegasi b, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus. As I have mentioned before, it was the first exoplanet to be discovered orbiting a main-sequence star, the Sun-like 51 Pegasi, and marked a breakthrough in astronomical research. It is the prototype for a class of planets called hot Jupiter.
Figure 5: An artist's impression of 51 Pegasi b (centre) and its star (right).
References
Doppler Spectroscopy (2021, March 4). In Wikipedia. https://en.wikipedia.org/wiki/Doppler_spectroscopy#Procedure
Exoplanet. (2021, March 2). In Wikipedia. https://en.wikipedia.org/wiki/Exoplanet
Ferreira, B. (2021). 2019 Nobel Prize in Physics Awarded for Discovery of Exoplanet Orbiting a Solar-type Star. Accessed on: 3, March 2021, in: https://www.eso.org/public/announcements/ann19049/
Methods of Detecting Exoplanets (2021, March 4). In Wikipedia. https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Transit_photometry
NASA (2021, March 2). NASA Exoplanet Archive. https://exoplanetarchive.ipac.caltech.edu/
51 Pegasi b. (2021, March 4). In Wikipedia. https://en.wikipedia.org/wiki/51_Pegasi_b