What Are Exoplanets? Let’s Dive In!

Mankind’s relationship with the universe is best described by Buzz Lightyear’s catchphrase, “To infinity and beyond!”

In the age of innovation, each day brings us closer to what is beyond. Locating and studying exoplanets is one of the most recent efforts in space travel. Learn more about their properties and potential through these facts. It might make you feel like a speck in the universe – but perhaps in a good way.

An exoplanet is a planet located outside of our Solar System. The study of exoplanets is crucial, especially in finding planets similar to Earth that might host life.

Key Facts & Summary

• The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such.
•  The first exoplanet detected was Gamma Cephei Ab in 1998. It was not confirmed until 2003 though, and generally, it is believed that the two planets orbiting the pulsar PSR 1257+12 are the first confirmed discovery. The discovery was announced in 1992.
• The first planet discovered orbiting around a Sun-like star was 51 Pegasi b.
• In respects to a planet similar to Earth, the first was discovered in 2014 named Kepler 186f. It is also located in the habitable zone of its star.
• The habitable zone is a region around a star where liquid water could exist on the surface of a solid planet.
• All the known exoplanets fall into categories depending upon their size, mass, and orbital positions.
• The closest star system and closest planetary system to the solar system is at 4.37 light-years away. It is named the Alpha Centauri system.

• If life is discovered on other planets, then the panspermia hypothesis would greatly become more likely the answer on how life on Earth was established.
• There are over 4.000 exoplanets that have already been confirmed.
• Almost 5.000 other exoplanets are candidates waiting to be approved.
• Over 3.000 planetary systems have been discovered.
• About 700 systems have more than one planet.
• There are about 5 methods used for the detection of exoplanets. A sixth one has been implemented recently.
• In 2009 NASA launched a spacecraft named Kepler, specifically to look for and detect exoplanets.
• Among exoplanets, rogue planets have also been detected. They are planets that have been pushed out of their star system.

Since our knowledge of the Universe has expanded, searching for planets outside our Solar System has been a priority in the search for extraterrestrial planets and new habitable worlds that might sustain us.

The question of whether or not we are alone in the universe, or if there are other places such as Earth, may have well begun since our world has been mapped, and mostly explored. The first possible evidence of an exoplanet was noted in 1917, but it was quickly dismissed.

As technology progressed, many other possible discoveries of exoplanets were proposed as early as 1988 but were only later confirmed. In 1992 the first detection of an exoplanet was finally confirmed. The radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.

Ever since then, the discovery of exoplanets became somewhat of a priority, and naturally, a great interest to humanity. Technology progressed and new methods of detection were implemented specifically for this task.

Methodology

The detection of an exoplanet is a difficult task, mostly because exoplanets are outshined by the stars they orbit.  To circumvent this, about 5 primary methods of detection have been developed in order to find these planetary bodies.

• Radio velocity: a planet causes a star to wobble.
• Direct imaging: where the glare of stars is blocked in order to see objects nearby.
• Astrometry: the study which observes the star’s movements in relation to nearby stars.
• Gravitational microlensing: the light of the star is observed if it is dent by a planet’s gravitational pull.
• Transit: This is the most prolific form of finding exoplanets – The method in which a planet passes between its star and Earth, thereby dimming its star’s light.

Almost 97% of all the confirmed exoplanets have been detected through these indirect techniques of detection. Recently, the techniques of singular optics have been applied in the search for exoplanets.

Formation

It is theorized that planets form within a few to tens of millions of years of their star-forming. Observations of different systems of varying ages will help us to better understand how our own system developed. Generally, planets form in a gaseous protoplanetary disk. They accrete hydrogen/helium envelopes. 

These envelopes cool and contract over time and depending upon the mass of a planet, some or all of the hydrogen/helium is eventually lost into space.

Numbers

There are 4.099 exoplanets that have already been confirmed with 4.729 other exoplanets being candidates waiting to be approved. These exoplanets are divided and located in 3.045 confirmed star systems. About 700 of these systems have more than one planet. These numbers represent the exoplanets discovered up until 2019.

It is speculated that there are over 40 billion exoplanets present in the Milky Way galaxy alone, 11 billion of them orbiting Sun-like stars. In the Universe, it is speculated that there should be about 1-2 maybe even more planets for almost every star.

Considering that there are more than 200 billion galaxies out there, the numbers surely contain a lot of digits.

General Features – Types of Exoplanets

The types of exoplanets discovered have features similar to the planets of our solar system. They can be categorized in:

• Neptune-like – over 1.300 have been confirmed.
• Super-Earth – over 1.250 have been confirmed.
• Gas Giant – over 1.200 have been confirmed.
• Terrestrial – over 160 have been confirmed.
• Unknown – more than 10 

When it comes to the gas giant exoplanets, super-Jupiter’s are planets similar to Jupiter but much bigger. There are also Mini-Neptune’s often referred to as gas dwarfs who are usually smaller than Uranus or Neptune.

When it comes to super-Earth exoplanets, they usually have a greater mass than Earth and usually, do not imply anything about the surface conditions or habitability.

This type of planet is the only one that isn’t present in our solar system but astronomers believe that we may have had one in the past that was expelled from our system or collided with a gas giant.

Some of the first exoplanets discovered were the PSR 1267+12 B and C which were pulsar planets. Found in 1992, they orbit the rapidly spinning remains of a massive star that exploded as a supernova.

One of these planets is a Super-Earth planet. When it comes to the first Earth-sized planet, Kepler 186f was the first to be discovered in 2014. It is circling in the habitable zone of its star.

In the end, there are many different types of planets ranging from all sizes, shapes, colors, atmospheres, orbits, and maybe unique features until proven otherwise.

Eccentricity

Many of the exoplanets discovered so far have a higher orbital eccentricity than the planets of our Solar System. Those with low orbital eccentricities usually orbit very close to their star and are tidally locked to it.

This for the moment proves that our Solar System’s unusually low eccentricity is rare. Low eccentricity is needed for habitability.

Habitability

One of the main goals of exoplanetology is to find life outside our Solar System. As the field grows along with technology, someday maybe, we will find extraterrestrial life. Many exoplanets similar to Earth have already been discovered. They orbit their star in the habitable zone and are usually given the Kepler name and a designation.

Future plans

There are many projects going on about the discovery and studying of exoplanets. New telescopes, both on Earth and space are prepared to be launched or have been launched already. In the next years with technology improving, who knows what planets await to be discovered. The Kepler mission is still on-going to this day.

Did you know?

• More than 22% of Sun-like stars have Earth-sized planets in their habitable zone.
• There is at least one planet on average per star with about 1 in 5 Sun-liked stars having an Earth-sized planet in the habitable zone.
• There could be about 1 trillion exoplanets in the Milky Way.
• At least one exoplanet has been discovered to have an exomoon. It was discovered in 2013 orbiting the Kepler-1625b exoplanet.
• Several exoplanets have been discovered to have atmospheres. The first to be observed was HD 209458 b in 2001.
• There is an exoplanet that has a tail like a comet. KIC 12557548b is an exoplanet very close to its star, and because of this, it leaves a trail of cloud and dust.
• Rogue planets are everywhere, and it is theorized that there could be 2 rogue planets per star in the whole universe.

• K2-18 b is probably the best candidate for habitability presently known.
• The first picture that was ever taken of an exoplanet happened in 2004. 2M1207b is a planet with five times the mass of Jupiter.
• The easest type of exoplanets to find are hot Jupiter’s since they’re massive and fast, making their signal easier to detect.
• The smallest exoplanet that has been found is named Kepler-37b. It is smaller than Mercury.
• It is quite possible that there are more planets in the universe than stars.
• Exoplanets orbit any kind of star.

Earth-like exoplanets could support alien life.

It might sound crazy to us, but believers of the extraterrestrial aren’t exactly wrong. In fact, a handful of the confirmed exoplanets lie within the habitable zone.  Also called the Goldilocks zone, this refers to the range where a planet’s distance from a star can still support life. Exoplanets within a star’s habitable zone have the potential to carry life under the right atmospheric pressure. Scientists estimate that the nearest Earth-like exoplanet could be 12 lightyears away from Earth.

The darkest planet in the known universe is an exoplanet.

For one, the darkest planet known to man is the TrES-2b exoplanet. This exoplanet reflects less than 1% of its host star’s light. As a result, TrES-2b reflects less light than coal or black acrylic paint. Hot Jupiter exoplanets typically have a lot of sodium and potassium in their atmosphere, hence the dark color. However, the exact chemical compound that makes TrES-2b darker than the rest remains undiscovered.

Astronomers identified the color of an exoplanet for the first time in 2013.

People might have started studying extrasolar planets in the 1990s, but it wasn’t until 2013 that an exoplanet’s color was properly identified for the first time. Astronomers measured the albedo of HD 189733b to find that it was a deep, dark blue. 

From there, they were also able to identify the colors of other exoplanets. GJ 504 b was calculated to have a magenta color, while Kappa Andromedae b would appear reddish up-close. Astronomers believe that helium planets are either white or grey.

10 billion Earth-like planets could exist within the Milky Way.

If we’re talking about the rest of the universe, this number expands tenfold. Kepler 22b made history in 2011 as the first exoplanet found within the habitable zone. When the news broke out, people were quick to label it as a “super Earth” that we could possibly live in. However, with a distance of 587 light-years from Earth, it would take eons to reach this exoplanet. Currently, it is still under observation.

NASA’s Kepler Space Telescope discovered most of the exoplanets we know now. 

First launched in 2009, NASA launched Kepler to locate extrasolar planets. Initially, the Kepler mission was supposed to last only 3.5 years. However, it wasn’t until 2018 that NASA retired the Kepler project after the telescope got incapacitated. All in all, Kepler observed 530,506 stars and located 2,662 exoplanets. Now, NASA’s Transiting Exoplanet Survey Satellite (TEES) continues the search for exoplanets, discovering 1730 candidates and 41 confirmed exoplanets so far.

Stars with higher metallicity are more likely to host exoplanets.

Most physical matter in the universe consists of hydrogen and helium. Metallicity is the term astronomers use to describe all elements other than hydrogen and helium. 

According to data gathered from the Kepler telescope, stars with more varied elements are more likely to have extrasolar planets orbiting around them. 

Gravitational microlensing uses other stars to spot an exoplanet.

Gravitational microlensing is similar to radial velocity, except this method involves another star aside from the exoplanet’s host. When one star passes in front of another star, its gravity acts like a lens that magnifies the other star’s light. If the “lens” star has a planet orbiting it, the exoplanet’s mass increases the magnifying effect. This is what astronomers used to locate over 20 exoplanets. 

Most exoplanets were discovered through radial velocity. 

The general rule of thumb for identifying exoplanets is to observe the star’s motion. Also called the Doppler’s wobble, this method is the most successful one yet with almost 400 exoplanets discovered. A star’s radial velocity changes when it has an exoplanet exerting gravitational pull around it. As such, the star will look like it’s “wobbling,” hence the name.

The transit method is the simplest method to find exoplanets.

The transit method is one you can see for yourself. Twinkle Twinkle, Little Star takes on a new meaning in terms of the transit method. Observed from the Earth, astronomers estimate the orbits and masses of extrasolar planets through the amount and frequency of a star’s “twinkling.”

About 4374 potential exoplanets are still under observation. 

Although it is not uncommon to find astronomical bodies outside our Solar system, locating extrasolar planets is a little trickier. First of all, these astronomical objects have to meet certain criteria before being confirmed as exoplanets. 

First off, the object’s mass must never exceed 30 Jupiter masses. Second, the planet must not be free floating or orbiting around nothing. Planets that don’t orbit around a star are instead classified as rogue planets. Aside from these points, exoplanet candidates undergo lengthy periods of validation.

Sources:

https://kidadl.com/facts/exoplanet-facts-formation-discoveries-and-interesting-trivia

https://nineplanets.org/exoplanets/

https://facts.net/exoplanets-facts/

https://en.wikipedia.org/wiki/Lists_of_exoplanets

https://www.theatlantic.com/science/archive/2022/03/nasa-exoplanets-habitable-zone/627155/