What Is The Difference Between Planets And Exoplanets?

Planets are celestial bodies that circle the sun. They are usually large enough to be visible from Earth.

Exoplanets are planets outside our solar system. They orbit their host star at distances much greater than Mercury orbits the Sun.

Exoplanets are discovered using telescopes.

These include NASA’s Kepler space telescope, Hubble Space Telescope, and ground-based observatories such as Keck Observatory, Gemini North, Subaru, and ESO’s Very Large Telescope.

There are over 2,500 exoplanets known today. Most of them are too far away to see directly.

Astronomers estimate that there are billions of exoplanets out there. Some of these planets are similar to Earth. Others are completely alien worlds.

We look at planets and exoplanets in closer detail and analyze the differences to help you distinguish one from the other. Let’s jump in. 

What Is A Planet?

A planet is a celestial body that circles a star or sun like our own. It has its gravitational pull.

This makes it difficult for smaller bodies (like asteroids) to pass by without being pulled into orbit.

Planets can have different sizes and compositions. For example, Jupiter is larger than all the other planets combined.

It also has more mass than any other planet.

Planets can be classified based on size and composition.

There are four main categories: gas giants, ice giants, terrestrial planets, and super-Earths.

Gas Giants

These planets are mostly made up of hydrogen and helium. Gas giants are often referred to as Jupiters.

They range in size from about twice the diameter of Earth to almost 10 times the size of Jupiter.

Ice Giants

Ice giants are composed mainly of water and carbon dioxide. They are sometimes called Uranus and Neptune.

They are slightly smaller than Jupiter but still very big compared to the other types of planets.

Terrestrial Planets

This category includes rocky planets with lots of iron and silicates. Examples include Mars, Venus, Mercury, and Earth.

They are relatively small compared to gas giants and ice giants.

Super-Earths

Super-Earth refers to a planet with a radius between 1.5 and 4 times that of Earth. These planets are not as dense as Earth but they are still solid.

However, they are significantly smaller than gas giants and ice giants and are considered “mini-Neptunes.”

What Are Exoplanets?

An exoplanet is an object orbiting a star at a great distance.

It does not necessarily mean that the object is a planet. An asteroid could also orbit a star at this distance.

The term exoplanet was coined in 1992 when astronomers first spotted a planet orbiting another star.

Since then, many thousands of exoplanets have been found. Today, we know of over 2,500 confirmed exoplanets.

Many more are expected to be discovered soon.

Most exoplanets are detected indirectly through the Doppler effect.

This means that the light coming from the star changes frequency depending on how fast the star is moving toward or away from us.

When an exoplanet passes in front of the star, the starlight gets shifted toward higher frequencies.

Conversely, if the star moves away from us, the light shifts back down to lower frequencies.

Exoplanets are usually much farther away than the stars they orbit. To date, only about 30% of exoplanets have their orbits measured.

Most of them are too far away to measure directly. Instead, scientists use the Doppler shift method to estimate their orbital periods.

Exoplanets come in two basic forms: hot and cold. Hot exoplanets orbit close to their host star. Cold exoplanets orbit further out.

Hot exoplanets are thought to be rocky planets similar to Earth.

Scientists think that these formed closer to their parent star and migrated outward due to tidal forces.

Cold exoplanets are probably icy worlds like Saturn’s moon Titan.

Some scientists believe that these planets were captured by their host stars after migrating inward.

Others think that these planets form farther out and migrate inward.

Differences Between Planets And Exoplanets

Planets and exoplanets are different in several ways. Here are some differences you should keep in mind.

Size

A planet is a sphere. A planet can be large or small. A super-Earth is larger than Earth but smaller than Neptune.

A planet has a defined surface. On Earth, the surface is where land meets the ocean.

There may be no surface for an exoplanet because it might be covered by liquid water.

Orbit

Planets move around their suns in elliptical paths. Their orbits change over time.

Exoplanets do not move relative to their suns. They move around their suns and they orbit stars.

Mass

Planets are made up mostly of rock and metal.

They have masses ranging from less than one Earth mass (1.099 x 10^24 grams) to almost ten Jupiter masses (1.327 x 10^27 grams).

Exoplanets are made up of gases such as hydrogen and helium.

Their masses range from less than 1 percent of Earth’s mass to hundreds of times greater than Earth’s mass.

Temperature

Planets are always warmer than their suns. However, the temperature difference between a planet and its sun varies.

For example, Mercury is closest to the Sun, and so it receives the most heat.

Venus is farthest from the Sun, and so it experiences the least heat.

The temperature of an exoplanet depends on its distance from its host star.

As an exoplanet gets closer to its star, it heats up. As an exoplanet gets farther away from its star, it cools off.

The temperature of an exoplanetary atmosphere also depends on whether the planet is orbiting a giant star or a dwarf star.

Giant stars give off more energy than dwarf stars.

So, the atmospheres of exoplanets orbiting giant stars will be hotter than those orbiting dwarf stars.

How Do We Know If An Object Is A Planet Or An Exoplanet?

The first step in identifying if an object is a planet or not is determining whether it is orbiting a star. To do this, astronomers use astrometry.

Astrometry is a form of astronomy that involves measuring the position of celestial objects relative to each other.

Astrometry measures how fast an object moves against background stars.

When we observe an object moving against the background stars, we know that it is orbiting a star because the object will appear to move around the center of the star.

Astronomers observe the object while it passes through a specific part of the sky.

By comparing where the object was when it passed through the same area of the sky again later, astronomers can calculate how much the object moved during the period.

To determine if an object is orbiting a star, astronomers must compare two observations taken over a short amount of time.

The difference in these measurements tells us how far away the object is from the star.

If the distance between the object and the star is close enough, then the object is likely orbiting the star.

If the distance is too large, then the object is most likely not orbiting the star.

Final Thoughts

We hope by reading this article you have learned all about planets and exoplanets and how although they both are objects in the universe, they are completely different from each other.

Planets are celestial bodies that circle a star or sun, whereas exoplanets are objects orbiting a star at a great distance.

Both still have room for extra-scientific research, and we are still discovering new facts about space every day, including planets and exoplanets!