Artist’s impression of ten hot Jupiter exoplanets [Credit: ESA/Hubble & NASA]

We live in a Solar System that has a variety of planets: four different rocky ones, giant Jupiter, ringed Saturn, two ice giants and a selection of moons and dwarf planets. Since we have so many planets, astronomers had long wondered how common they might be elsewhere in the Galaxy. However, before 1992 no one had been able to find one.

The first planets discovered beyond the Solar System, i.e., exoplanets, were quite a surprise. In 1992 Aleksander Wolszczan and Dale Frail were studying the pulsar PSR B1257+12 when they found that it had two planets.

But a pulsar is a fast rotating neutron star, the remnant of a giant star that exploded in a supernova. Considering the power of a supernova, who would expect to find a surviving planetary system? No one. Yet it turned out that PSR B1257+12 has three of them, and more pulsars with planets have been discovered since then.

A planet around a sunlike star would take a few more years to find. The first discovery, and most of the early discoveries, were made using Doppler spectroscopy. (It's also known as the radial velocity method or even the wobble method.)

Doppler spectroscopy – how does that work?
Strictly speaking, a planet doesn't orbit a star. Their mutual gravitational force holds them together while they both orbit their common center of gravity. Obviously, the star is going to be the main force here, but the planet also has an effect on the star that may sometimes be detected by observing the light spectrum.

If a body is moving towards us, its light becomes bluer, and if it's moving away, it gets redder. You can see in this diagram how an orbiting planet gives its star a little push or pull. If it's in our line of sight, sensitive equipment and enough viewing time may detect the wobble the planet gives the star.

One exoplanet – one shock
From the experience of our Solar System, everyone knew that a giant planet like Jupiter couldn't be located close to its star. So when Michel Mayor and Didier Queloz discovered just such a planet in 1995, it defied what everyone knew. It was closer to its star than Mercury is to the Sun, and zipping around its star every four days. You can see the planet's effect on 51 Pegasi in this graph.

Mayor and Queloz were awarded the Nobel Prize in Physics in 2019 for the discovery of planet 51 Pegasi b, now named Dimidium. It turned out to be the first of many planets which are classified as hot Jupiters.

Near and far
WASP-12 b is a hot Jupiter that's twice the size of our Jupiter, and more massive. Yet it takes just over a day to get around its star. Because of its closeness to the star, the same side of the planet always faces it, and the temperature is hot enough to melt most metals, including carbon steel. In addition, heat flows from the day side to the cooler night side, producing strong winds.

Being so close, the star's tidal forces are also pulling the atmosphere away and stretching the planet into an oval shape. The tidal interactions cause the planet's orbit slowly to decay. Some day the planet will be part of the star.

We might imagine that with all the energy WASP-12b gets from its star, it would be a bright object. However, instead of reflecting it, almost all of its sunlight is absorbed, leaving the planet described as “black as asphalt”.

Interestingly, while WASP-12 b is on the way to becoming part of its star, 2MASS J2126-8140 is quite the opposite of a hot Jupiter. It's many times bigger than Jupiter or WASP-12 b. However, it's so far away from its star, astronomers originally thought it was a free-floating planet. It takes some 900,000 years to complete one orbit around its red dwarf star.

Puffy Jupiter
Not every hot Jupiter is as massive as Jupiter. Tres-4 b, orbiting its star in 3.5 days, is 1.7 times the size of Jupiter, and one of the biggest known planets. Yet Jupiter's density is 70 times greater than Tres-4 b, which has the density of cork. Tres-4b is a puff planet.

There are other puff planets, including some known as super puff planets. These have the density of cotton candy, and are Neptune sized. Puffy planets are still something of a mystery.

More suns than Tatooine
In the Star Wars saga, young Luke Skywalker lived on the planet Tatooine, which had two suns. He was not impressed by the suns and rated the planet as the most boring place in the galaxy.

However, astronomers are fascinated by such star systems. And a team of researchers at the Harvard-Smithsonian Center for Astrophysics found a planet in the star system KELT-4 which has three suns. Three suns wouldn't have impressed Luke, and the planet KELT-4A b is one to avoid. It's a hot Jupiter orbiting the star KELT-4A about every three days. KELT-B and KELT-C orbit each other every 30 years and the pair orbit KELT-A about every four thousand years.