Planets

There are only eight planets in our solar system. But there are believed to be many quintillion planets in the universe at large. These are generally known as exoplanets – planets outside our own system.

Astronomers try to group exoplanets into different categories. There are several different approaches to this. But NASA uses a simple system that focuses on the size of each planet.

This system has four different categories for planets: Terrestrial, Super-Earth, Neptune-Like and Gas Giant.

Terrestrial planets are Earth-sized or smaller, and usually made of solid materials like metal and rock. The terrestrial planets in our own solar system are Mercury, Venus, Earth, and Mars.

A Super-Earth, on the other hand, is larger than the Earth, but smaller than Neptune. They're also typically solid and rocky, but they come in lots of shapes and forms. Watery worlds, icy worlds, rocky worlds... there are lots of possible Super-Earths.

There isn't a Super-Earth in our solar system, but astronomers have identified many in other star systems.

Neptune-Like planets are larger than Neptune, but smaller than Saturn. Typically, they have dense, heavy cores, surrounded by thick, gaseous atmospheres with lots of hydrogen and helium.

In our own solar system, the Neptune-Like planets are Neptune and Uranus.

Last but not least, we have Gas Giants. These are the same size (or larger) than Saturn, and have gaseous surfaces. Our own gas giants are Saturn and Jupiter.

Although Jupiter and Saturn are in the outer regions of our solar system, some Gas Giant exoplanets orbit close to their stars, and reach extremely hot temperatures. Astronomers call these Hot Jupiters.

Hot Jupiters present us with a puzzle. What are these massive planets doing so close to their stars?

The planets could have formed close to their stars. But young stars are typically explosive and turbulent – these conditions would probably have prevented the formation of planets as large as these ones.

Instead, maybe these planets originally formed further out from the star, where conditions were more stable. Then, over time, they migrated inwards, until they were significantly closer to the star.

But what would cause them to migrate like that, while the Gas Giants in our own solar system stayed in a stable orbit? As things stand, Hot Jupiters are one of the many things that astronomers don't fully understand.

Finding exoplanets – let alone studying those exoplanets – can be extremely challenging for astronomers.

The main issue is brightness. Planets are small and dim, and the light from their stars tends to drown them out. Because of this, the chances of spotting an exoplanet through a telescope are extremely low.

Astronomers can use coronagraphs – a device which blocks light inside telescopes. A coronagraph can blot some of the light from a star, and make the planets easier to see.

But more commonly, astronomers use two different methods to search for exoplanets: the radial velocity method and the transit method.

The radial velocity method is based on gravitational pull. Stars exert a strong gravitational pull on their orbiting planets – but planets also exert a much smaller gravitational pull on their stars.

This pull can be enough to make a star wobble very slightly, and this wobble shifts the wavelength of light emitted by the star. We can use this shift as evidence of an exoplanet.

We have so far discovered 936 planets using this method. Massive planets, like Gas Giants, are easiest to find using this particular method, because the pull on their host star is strong.

The transit method also looks for shifts in the light from a star. But this time, the shift isn't caused by a planet's gravity.

Instead, it's based on the following: when an exoplanet passes in front of a star, the star briefly gets slightly dimmer. It’s like a much subtler version of a solar eclipse, which results from the moon passing in front of the sun.

When a planet passes in front of a star, light will also travel through that planet's atmosphere. By analyzing this light, we can even learn about the chemical composition of this atmosphere.

The transit method is very useful when searching for exoplanets. So far, we have found 3879 planets using this method. That's four times as many as the radial velocity method.

Discovering that planets exist is one thing. But how do we learn more about them? Again, astronomers have devised some clever techniques.

As we've already mentioned, we can study atmospheres by looking at light that shines through them. Different elements and compounds absorb different wavelengths. This technique is called transit spectroscopy.

We can also graph the dimming of light when a planet crosses a star. This graph is called a light curve. The larger the planet, the deeper the light curve, as it blocks more light from its star.

The mass of a planet can also be measured using the radial velocity method. A larger planet will generally cause a greater wobble to the star.

In 2009, the Kepler Space Telescope was put into orbit around the Earth. It had one main purpose: to search for Earth-sized exoplanets.

It used the transit method, and in nine years of operation, it discovered more than 2600 planets. It changed how we view the universe, revealing that there may be more planets in the universe than stars.

In 2021, the James Webb Space Telescope was launched into space. It specializes in exoplanet detection and characterization. Hopefully, over the next few years, it will help astronomers to learn even more about planets other than our own.