Discovering Exoplanets: What, How, and Why

From the Lecture Series: A Field Guide to the Planets

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

When exoplanets were first discovered, they put a red X on some scientific beliefs of their time. They were big surprises in some cases, for example, when it was known that they are 10 times bigger than Jupiter but eight times closer to their star than Mercury is to the Sun. How were these celestial bodies discovered? And, do we now know everything about them?

Exoplanets around a distant star.
Exoplanets vary greatly in different specifications, but they all orbit a star somewhere in the universe. (Image: Dotted Yeti/Shutterstock)

An exoplanet is a planet that orbits a star in a stellar system, just like how the planets in our solar system do. However, they are still mysterious in some ways. How were they formed? There are theories, but none is thoroughly accurate in all conditions.

Hot Jupiters

Hot Jupiters, as the name suggests, are giants much hotter, much more massive, and bigger than Jupiter. They were either formed outside their stellar systems and slowly moved in, or created when the system was in the making.

Hot Jupiters were among the first planets discovered around a pulsar—the super-fast spinning neutron star. The reason was the biases in the methods used to discover exoplanets. It was easier to spot very massive planets orbiting very close to their parent stars.

Learn more about mighty Jupiter, the ruling gas giant.

The Center of Mass

The first exoplanets were discovered using the radial velocity, or RV, method. The method relies on one fact: planets not only orbit static stars with a fixed location, but the planet and the star can also both orbit the center of mass of the system, i.e., the location of the averaged mass in the system.

Quasar surrounded by an orbiting accretion disk of gas, supermassive star with X-rays and electromagnetic radiation.
The planets and the star all orbit the center of mass, which is always closer to the more massive object. (Image: Jurik Peter/Shutterstock)

The more massive an object gets, the closer it pulls the center of mass. Normally, as stars are much more massive than the other planets in the system, the center is within the star, but not at its center. This is evident in the wobbly motion of the stars.

In the RV method, the light coming from the star is the base. That light gets blueish when the star is moving toward us and reddish when it moves away from us. The color-shifting of the light determines the velocity of the star’s wobble.

The greater the mass of a planet is, the greater its effect on the wobble will be. For example, the Earth causes the Sun to wobble with a speed of 10 centimeters per second. On the other hand, Jupiter, with a mass 318 times greater than the Earth, makes the wobble about 12 meters per second. The period of the wobble is proportionate to the planet’s orbit.

Thus, the wobble can determine the minimum mass of the planet, its orbital period, and the distance from the star.

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

The RV Method

Despite all the advantages of the RV method, it cannot specify the full mass of the planet unless it is orbiting directly in our line of sight, which is very rare. The orbit inclination can lead to receiving a weaker signal than the actual signal amplitude.

The method can determine the ellipticity of the orbit as well. The method helped discover over 800 exoplanets by 2019. That is about 30% of known planets at that time.

Learn more about comets, the Kuiper Belt, and the Oort Cloud.

The Transit Method

The transit method is another method of detecting exoplanets while measuring the inclination of the orbit and even the planet’s radius. The transit method discovered its first exoplanet in 1999: HD209458b, a hot Jupiter that was later nicknamed Osiris.

Osiris was in the Pegasus constellation, about 160 light-years away from Earth. The RV method had already discovered it, but the transit method helped determine that its actual mass was about 70% of Jupiter.

The method confirmed that exoplanets are actually planets, and they are not too massive for it. With RV there was the possibility that a brown dwarf has been detected, but it orbits the star at an extreme angle that minimizes the detectable wobble. When a gas giant gets as massive, almost 14 Jupiters in size, it is then a brown dwarf.

Dimming of the Light

Exoplanet gas giant hot Jupiter or brown dwarf 3D illustration
The first discovered exoplanets were hot Jupiters because it was easier to detect these massive giants. (Image: Freedom_Marussia/Shutterstock)

Each time a planet passes in front of a star, its light dims from a fraction of a percent to a few percent, depending on the size of the planet. As the orbit is repeated regularly, the dimming is also periodic, determining the planet’s orbital period.

Now, the RV method had determined a minimum mass, and the transit method, the radius. That was enough to calculate the density of an exoplanet for the first time.

Learn more about how the solar system family is organized.


The density of Osiris is 370 kg per cubic meter, which is too low even for a gas giant made purely of hydrogen. However, Osiris is a hot Jupiter orbiting almost nine times closer to its star than Mercury to the sun, with a surface temperature of about 1000°C. Thus, its density is lower than even Saturn.

Osiris’s atmosphere has oxygen, carbon, sodium, carbon monoxide, and water since every time it passes the star, it orbits some wavelength of the light it absorbed depending on the atoms.  The scattered redder or bluer starlight shows that its extended hydrogen atmosphere is being blown off the planet.

Exoplanets are slowly getting known through a combination of methods, but they still hold many surprises.

Common Questions about Exoplanets

Q: What are exoplanets?

Exoplanets are planets that orbit stars, and sometimes pulsars, in other stellar systems. Sometimes they give so much light that they are impossible to miss, and sometimes they disturb a pulsar’s signals.

Q: How was the first exoplanet discovered?

The first exoplanets were discovered when the ordinary and stable signal from a pulsar was disturbed. Scientists found some planets that orbited the pulsar.

Q: What are Osiris and the Transit Method?

The transit method is a method of detecting exoplanets while measuring the inclination of the orbit and even the planet’s radius. The transit method discovered its first exoplanet in 1999: HD209458b, a hot Jupiter that was later nicknamed Osiris.

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