The exoplanet discovery behind the 2019 Noble Prize in Physics explained


Last week the Royal Swedish Academy of Sciences announced the winners of the 2019 Nobel Prize in Physics. We were overjoyed to find out that this year’s most prestigious scientific award goes to three astronomers, James Peebles, Michel Mayor and Didier Queloz, for their “contributions to our understanding of the evolution of the universe and Earth’s place in the cosmos”. James Peebles will receive half of the prize in recognition of his “theoretical discoveries in physical cosmology”. Another half of the prize will go jointly to the Swiss astronomers Michel Mayor and Didier Queloz “for the discovery of an exoplanet orbiting a solar-type star” the scientists made in 1995. 



In October 1995, Michel Mayor and Didier Queloz discovered 51 Pegasi b, the first exoplanet to orbit a main-sequence star 51 Pegasi. You may know this planet as Dimidium, by the name the public have chosen for 51 Pegasi b in a 2015 vote organized by the International Astronomical Union. 

51 Pegasi is a Sun-like star situated 50 light years away from us. It is a bit older than the Sun, and is slightly bigger and heavier. Under some very dark skies this star can be spotted with a naked eye. It is located in the constellation Pegasus, the winged horse. Although the star resembles our Sun, the planet that orbits around it is nothing like any of the planets in the Solar System. 51 Pegasi b is what we now call a “hot Jupiter”, i.e. a gas giant planet, like our Jupiter only hotter, with extremely short orbital period. This planet is 50% larger than Jupiter, although less massive. It zooms around its host star once every 4.2 days at a distance so close that the temperature on the planet rises above 1000 degrees C. To compare, Mercury, the fastest planet in the Solar System, takes 88 days to orbit the Sun. The temperature on Mercury’s dayside can reach 430 degrees C.



51 Peg b, just like most other “hot Jupiters” later, was discovered using radial velocity method. The idea behind this method is fairly simple. As a star experiences a small gravitational tug from its planet, it begins to wobble, or more precisely move around the center of mass of the system. Astronomers can detect this motion by looking at the star’s spectrum. When the star is moving towards us, its light appears blueshifted, when it moves away, its light appears redshifted. 

Most ground-based exoplanet discoveries were made using the radial velocity method. In recent years a different method, called transit photometry, became the most successful way of searching for planets around other stars.


First, but not first

We must point out that 51 Pegasi b was NOT the first detection of an extrasolar planet, but the first detection of a planet around a main-sequence star. The very first exoplanet has been found 3 years earlier, in 1992, by the Polish astronomer Aleksander Wolszczan in collaboration with the Canadian astronomer Dale Frail. The scientists found two planets around a millisecond pulsar PSR B1257+12 by observing irregularities of its pulsations with the Arecibo Radio Telescope. The discovery shocked the scientific community. Wolszczan’s exoplanet orbited not a sun-like star at all, but a pulsar, a deadly fast rotating neutron star. That was most unexpected! Interestingly, the pulsar planets (i.e. exoplanets around pulsars) are extremely rare whereas exoplanets around “normal” stars are quite common. At the moment out of over 4 000 known exoplanets only 5 are pulsar planets. Astronomers suspect that planets around pulsars form differently than their “normal” cousins.

New questions

The discovery of this impossible oddball revealed something shocking: our theory of planetary formation is far from being complete! According to the “classic” theory the gas giants reside in the cold regions on the outskirts of planetary systems. So 51 Pegasi b shouldn’t have even existed! But later observations showed that hot Jupiters are quite common and that planetary systems are very diverse. It looks like it is our own Solar System that is odd after all!


More on the topic

Check out our other blog posts about the Nobel Prize winning astronomical discoveries 

Read the latest exoplanet news in our blog post

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