Pulsars, Exoplanets, and Hot Jupiters

From the Lecture Series: A Field Guide to the Planets

By Sabine Stanley, Ph.D., Johns Hopkins University

Hot Jupiters are a group of exoplanets that are gas giants just like Jupiter. Some theories have tried to discover and explain where they come from, but so far, evidence shows that they are giant exoplanets and can massively affect the system they inhabit. Read on to find out what else is connected to these giants.

A hot Jupiter class exoplanet, gas giant planet lit by an alien sun
Hot Jupiters are exoplanets many times bigger and hotter than our Jupiter. (Image: Dotted Yeti/Shutterstock)

Do planets with a surface temperature of 1300°F, wind speeds of over 5000 miles per hour, and molten glass rain belong to sci-fi movies? No, they are very real. What about a world only twice the size of Earth’s Moon, orbiting the carcass of an ancient star that takes only 1/1000 of a second to complete a rotation? Also real. There is even a solar system with four planets, each about 10 times more massive than Jupiter! What are these objects?

What Are Exoplanets

Exoplanets are planets around other stars. They are everywhere in the universe, and each has unique characteristics. They show that our solar system is typical, and there are similar systems around. At the same time, they show that the variety of planets and systems is far beyond the solar system.

The first exoplanets were discovered in 1992: two planets, each around four times more massive than Earth were orbiting a pulsar called “Lich”. The third planet of the same system was only 2% the mass of Earth. The system is 2,300 light-years away from the solar system in the Virgo constellation. But, what exactly is Lich?

Lich and Its Planets

A pulsar is a small, hyper-dense, frantically rotating neutron star and is strongly magnetized. Lich is a pulsar originally designated PSR B1257+12, with a mass of about 40% larger than the sun and a radius of only 10 kilometers. Lich’s spins in every second emit electromagnetic radiation.

Scientists discovered the planets orbiting Lich when its signal was interrupted. Phobetor orbits Lich every 98 days at an orbital distance of 0.46 astronomical units. Next, is Poltergeist, named after the ghostly spirit, orbiting every 67 days at an orbital distance of 0.35 astronomical units. The closest one is Draugr, only about twice as massive as Earth’s Moon. It orbits at only 0.2 astronomical units, with an orbital period of 25 days. It was a surprise to find planets around a pulsar.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.

How Do Pulsars Form?

Pulsars form when large stars run out of fuel and gravitationally collapse in on themselves, resulting in a supernova explosion. After the explosion, a super-dense ball of neutrons is left at the center, spinning very fast.

This spinning neutron ball blows away everything within its orbital reach, so, it is really surprising to find planets orbiting it. Thus, planets around Lich were not there when Lich was a normal star. They were created later from a disk that formed after the supernova. Are there planets orbiting normal stars as well?

Learn more about Mercury, the extreme little planet.

Main Sequence Stars

Stars that supply ordinary light to their system are called “main sequence” stars. The first main-sequence system was discovered in 1995 and was 50 light-years away. As the star was very similar to the Sun, the environment of the stellar system must also have been similar to the solar system.

This star, 51 Pegasi, has a planet called Dimidium. Dimidium is half the size of Jupiter, 150 times more massive than Earth, and at an orbital distance eight times closer to 51 Pegasi than Mercury is to the sun, i.e., five astronomical units. Dimidium’s orbital period is only four Earth days. But, how can such a big planet orbit so close to a star?

Hot Jupiters

Dimidium was not the only giant discovered with a very short orbital distance, from about 0.04 to 2.5 astronomical units. From 1995 to 2000, 27 similar planets were discovered. Their surface temperature must be over 1000°C according to their situation. They ranged in size from half of Jupiter to over 10 times of Jupiter.

Pulsar highly magnetized, rotating neutron star
Pulsars are too strong to leave any planet around when they are forming, but once they do, they can absorb planets to orbit them. (Image: Jurik Peter/Shutterstock)

These giants with high temperatures are called “hot Jupiters”. Scientists used to think no such planet can exist, based on what they knew of planet formation from the solar system. They were wrong, but do they know how hot Jupiters are made?

Learn more about mighty Jupiter, the ruling gas giant

How Are Hot Jupiters Formed?

There are three theories that have tried to explain how hot Jupiters were formed. According to the first, they were made from protoplanetary disks much more massive than in our solar system. Thus, the planet cores were giant enough to come close to the stars and attract the gases before they blow away. From the physics standpoint, it is unlikely to have protoplanets of about 10 Earth masses accreted in a few million years, but scientists have found specific conditions for it.

Second, hot Jupiters might have formed outside the stellar system and slowly migrated into it due to gravitational drag forces with the remaining disk. However, in this case, the other planets would be blown away by the giant’s forces or be absorbed by it. Hence, rocky terrestrial planets cannot form in the system.

Third, the giant can form outside the system, but then get gravitationally perturbed by a massive object even farther out than the planet. Next, the giant’s orbit gets much more elliptical, resulting in tidal forces that damp the ellipticity of the orbit, eventually pulling it into a more circular orbit very close to the star.

Hot Jupiters are still not fully understood, and it will be long before experts are finally able to figure them out. But, what we are sure of is that they have their own class of planets.

Common Questions about Hot Jupiters

Q: What are pulsars?

A pulsar is a tiny, hyper-dense, furiously-rotating neutron star that is strongly magnetized. Planets and even hot Jupiters can orbit pulsars.

Q: What is an exoplanet?

Exoplanets are planets around other stars. Hot Jupiters are some familiar examples to people who study astronomy.

Q: What is a hot Jupiter?

Hot Jupiters are a class of exoplanets that are large planets, highly irradiated by their stars, with hotter surface temperatures than other gas giants, large masses, and close orbits.

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