What are Exoplanets & How Do We Find Them?
Last Updated: June 19, 2023
We all know “My Very Eager Mother Just Served Us Noodles” or something similar to remember the planets in our solar system, but what about other star systems? What classifies as a planet? How do we detect planets orbiting other stars?
Are planets common in the galaxy and universe or unique to our solar system? Do other planets look like ours? What do these findings mean for our understanding of our solar system and the universe? Let’s explore exoplanets!
What is a planet?
Many planets in our solar system were observed in ancient times as wanderers across the night sky, not following the paths of stars: Mercury, Venus, Jupiter, and Saturn. Please check out our previous article on the brightest planets in the night sky for more information on observing planets.
As our telescopes and math advanced, we were able to find Uranus and Neptune. We also found an odd astronomical object while trying to balance the physics of our solar system, looking for a final planet. While everything still didn’t add up to create what we saw in reality with this new astronomical object, we accepted this object into our planetary family as the ninth planet Pluto.
But… Pluto was weird. It didn’t quite follow all the similar characteristics of the other planets, but it wasn’t a moon, asteroid, comet, or meteoroid. In the early 2000s, we found hundreds of objects like Pluto in the Kuiper Belt, an icy asteroid belt past Neptune’s orbit, which created a dilemma: if Pluto was a planet, were all these Kuiper Belt Objects planets too? Debates followed and the astronomical community realized that there wasn’t an agreed-upon definition of what a planet is. So, they decided to set one, looking at the characteristics of the planets to determine what made them all similar to agree upon a common definition and determine if Pluto qualified.
In 2006, the International Astronomical Union (IAU) released its decision. A planet was an astronomical body that:
- Orbits its star, and therefore not another astronomical body like our moon orbits Earth
- Is big enough that it has enough gravity to force it into a roughly spherical shape and not a lumpy asteroid
- Is big enough to clear away other astronomical objects of similar size from its orbit, either pulling objects in to become moons and rings or pushing objects out of the way
How does Pluto measure up to this definition? Pluto does orbit the Sun and has enough mass that it has formed a roughly spherical shape. However, Pluto is not big enough to clear its orbit of other objects as it goes in and out of the Kuiper Belt. The IAU created a new designation for astronomical objects in the solar system that, like Pluto, hit the first two qualifications of being a planet, but not the third: a dwarf planet.
Is this definition agreed upon by the astronomical community and the world? Not completely. Some of the general public is frustrated by the decision which caused Pluto to go from planet to dwarf planet.
There are some scientists that think this definition needs tweaking, especially as we continue to find more objects in our solar system and beyond. The IAU’s definition is not set in stone for eternity. It is subject to the Scientific Method and will be updated if data shows the need for it over time, but for now, it provides an accurate system of classification for our solar system and others.
Are there planets outside our solar system?
As our technology and understanding of math and physics have expanded over the years, we have been able to observe more and more of the night sky and make basic hypotheses about the universe. We’ve long pondered the existence of other planets out there in the universe. Other planets would mean the potential for life and/ or future habitable homes. We call planets that orbit other stars than our own extrasolar planets or exoplanets.
But the question of finding and confirming planets outside of our solar system was difficult. If even observing faraway stars is difficult, how can we observe their planets, tiny in comparison to massive stars? You can fit 110 Earths across the diameter of the Sun and it would take over a million Earths to fill it. In addition, we need light to be able to observe something. All of our knowledge of the universe comes from analyzing the light coming from different objects.
Planets don’t produce their own light, simply reflecting the light from their star. The light from stars easily overshines light that a planet might appear to emanate.
How many exoplanets have we found so far?
As of June 2023, a total of 5,438 exoplanets have been found. Here’s a breakdown:
- 871 are Neptune-size;
- 1711 are gas giants;
- 1653 are super-earths;
- 198 are terrestrial;
- 5 are an unknown type, meaning we don’t have enough information to classify them.
This number is likely to change rather rapidly as scientist discover new exoplanets on a daily basis. Let’s work through the different methods that have been developed or are being developed to detect exoplanets, starting from the beginning and working our way up to today.
How do we detect exoplanets from Earth?
Method 1: Pulsar Detection
As we discussed in our previous article on how stars form, there are many ways for a star to begin to die, most often relating to how big they are. When a giant star’s core collapses, exploding its layers out in a supernova, the remnants of the core may become what is known as a neutron star, which is extremely dense. Some of these neutron stars give off regular pulses of radiation in the radio wavelengths. Some of these are so fast they are called millisecond pulsars, putting our best drummers to shame. These pulses are so regular, that if they are off, astronomers want to find out why to understand what could be causing the upset.
In 1992, astronomers were studying the pulsar PSR B1257+12, 2300 light-years away in the constellation Virgo, because every once in a while, something caused the pulses to be slightly off. Eventually, they figured out the reason: 2 rocky planets orbiting the pulsar that would interfere with the radiation, causing them to be slightly off. Due to their location around a pulsar, this method is classified as pulsar detection. Debris from the supernova explosion can reform into planets that still orbit the pulsar, but they are definitely “dead planets” due to the devastation from the supernova and the radiation from the pulsar. While they were the first exoplanets discovered, this did not confirm that exoplanets around main sequence stars (during their active lifecycle before they begin to die) could be detected.
Method 2: Radial Velocity
All objects with mass impact space-time and objects around it with their gravitational force. Large objects have a larger gravitational pull and impact, but even the small ones still have an impact. For instance, Jupiter’s massiveness impacts the set-up and formation/ evolution of our solar system, keeping the asteroid belt in place and even impacting our Sun gravitationally.
Astronomers noticed that some stars “wobbled” in their orbit, the wavelengths of their light stretching and contracting as they moved slightly closer and farther away from us. They hypothesized that these wobbles, these changes in wavelengths could be caused by gravitational tugs on the stars by their planets, affecting what is known as their radial velocity.
In 1995, the first exoplanet orbiting a main sequence star was discovered using this radial velocity method: 51 Pegasi b, a hot gas giant half the size of Jupiter but much closer to its main sequence star meaning it raced around its 4-day orbit, tugging on the star gravitationally and causing the star to wobble.
This discovery of the first exoplanet, jumpstarted the hunt, starting the “classical” period of planet hunting which primarily found “hot Jupiters” that had extremely tight and fast orbits to change the star enough to be detected.
Method 3: Transit Method
When observing, we need to consider how bright our target is, where it is in relation to us, and the path of light from it to us. What we see, particularly of objects within our solar system, is greatly affected by our location in the solar system and where the target is in relation to us.
Particularly, the inner planets can lie between us and the Sun since they are within Earth’s orbit, causing some interesting interactions. Similar to how our moon can pass in front of our Sun at just the right angle to block the light from the Sun in an eclipse, the inner planets can also pass directly within the path of light between us and the Sun. The difference is all about size and ratios. The Sun is much bigger than our moon, but it is also 93 million miles away, meaning that we are actually very lucky in that the ratios line up so that our moon appears as the same size as the Sun in our sky and can therefore cover it up when the orbits line up perfectly. Both Venus and Mercury are bigger than our Moon, but further away from us and closer to the Sun. Therefore, when they pass between us and the Sun at the right angle, they block some of the sun’s light creating a shadow across its surface, which we call a transit. We see the planet as a dark dot traveling across the Sun’s surface when observing them during a transit with proper filters to block out most of the Sun’s light. We wondered if we could use a similar method to find exoplanets.
We needed powerful telescopes to see the slight dip in light that would occur when a tiny planet crossed the light of the already tiny star in our night sky. In 1999, two separate research teams independently observed a planet passing across the star HD 209458 in the constellation Pegasus, proving the viability of the transit method and the value of developing telescopes that could utilize it. The Kepler telescope ushered in the “modern” age of planet hunting, launching in 2009. In orbit around the Earth to limit the atmospheric distortions that occur here on Earth due to the molecules in the atmosphere, Kepler stared at a single small patch in the night sky containing 150,000 stars for four years, observing all of them for a dip as the planet transitted its star in relation to our observation angle of it. Once a dip is detected, its orbital size can be detected from the time it takes to orbit once around the star and the mass of the star. Kepler was in operation until 2018, discovering exoplanets and capturing data that will be used for years to come, allowing us to identify more as time goes on.
NASA’s Transiting Exoplanet Survey Satellite (TESS) launched in 2018, continuing the transit method hunt for exoplanets, analyzing whole swaths of the sky. Other space-based telescopes like Spitzer and Hubble have also used the transit method to detect exoplanets.
In addition to simply detecting whether or not a planet is there, the transit method allows for more analysis of this light revealing information such as what is in the planet’s atmosphere. Just like when you use a prism to reveal the color spectrum hidden in white light, scientists can analyze the bands of color and wavelengths to reveal what molecules are present using a method called transit spectroscopy. When the light passes through the atmosphere of the planet transiting it, it reveals where it’s been in comparison to the light not traveling through that atmosphere. Hubble has detected helium and water vapor in the spectrographs of exoplanets and the recently launched James Webb Telescope is providing even more data.
Method 4: Gravitational Microlensing
Einstein was the first to describe gravity’s ability to warp and bend starlight because gravity warps spacetime around heavy objects, forcing light to travel around these wells created by heavy objects like stars.
There are stars closer to and farther away from us. Even ones that appear right next to each other or about the same brightness are not necessarily so. Their light will be affected as viewed from here on Earth will be dependent on how bright it actually is and how far away it is from us. Gravity complicates it a bit more. The gravity of a star in the foreground (closer to us) will magnify light from a background star (further away from us and from the star) that passes behind it because of how the light moves around the star. If the foreground star has a planet, the star will appear as a spike in light intensity to a properly positioned telescope as the background star goes by and the planet will appear as a second, smaller spike.
While this gravitational microlensing method is currently being used by ground-based telescopes, it will be utilized by the future Nancy Grace Roman Space Telescope. This method is still relatively new and not as common as the two previous methods.
Method 5: Direct Imaging
As our imaging equipment and analysis instruments have improved in recent years, our ability to directly image targets has increased dramatically. A single pixel of light from say the James Webb Telescope contains a lot more than from a smaller ground-based telescope. The more light we can collect, the better a direct image will be just as a zoom lens on a digital camera is able to collect a lot more light than the tiny lens most cameras come with.
Currently, direct imaging of exoplanets is mainly from giant planets recently formed and are therefore still hot and emitting light. A great example is the 2017 animation of 4 exoplanets around the star HR 8799, from astronomers Jason Wang and Christian Marois using images from Hawaii’s Keck Observatory as tiny pinpricks of light moving around the bright star.
The next generation of space telescopes will utilize two technologies currently under development to expand the potential of direct imaging:
- Coronagraph: a system of masks, prisms, and detectors within the telescope to dim the overwhelming light from the star and help detect the dimmer light of exoplanets around it. It utilizes self-flexing/ deformable mirrors that adjust to and compensate for tiny flaws in real-time as the telescope captures the light traveling light-years to reach us.
Starshade: a system outside of the telescope that also works to dim starlight. The sunflower-shaped spacecraft, as large as a baseball diamond, will unfold in space for easier transport similar to the James Webb design, and be parked away from a space telescope to block starlight and reduce any additional stray light.
Method 6: Machine Learning
In a stunning convergence of astrophysics and artificial intelligence, researchers at Google AI and the University of Texas at Austin have made a groundbreaking discovery. They’ve unearthed an eighth planet orbiting Kepler-90, a Sun-like star over 2,500 light-years from Earth. But the real story is in the ‘how’.
They utilized a machine learning approach, leveraging a neural network—a computer system modelled after the human brain. The researchers trained this network to recognize exoplanets by sifting through a colossal amount of data from NASA’s Kepler Space Telescope, known for its four-year trove of 35,000 potential planetary signals.
The machine learning model, once trained on 15,000 previously vetted signals, successfully distinguished true planets from false positives 96% of the time. It was then directed to scrutinize the weaker signals in star systems already known to house multiple planets.
This innovative use of machine learning is not just a testament to the prowess of artificial intelligence, but it also marks a paradigm shift in how we explore the cosmos, proving that even the faintest whispers of distant worlds can’t evade the powerful sieve of machine learning
What are the current numbers? How common are Exoplanets?
So, now that we’ve established what a planet is, what an exoplanet is, that we have detected them and how we detect them, what’s the conclusion? Are planets common in star systems? How many have we found so far?
Simply put, planets are very common! In fact, it seems that most stars have at least one planet orbiting around them. The first multi-planet system was discovered in 1999 when two research teams independently announced the discovery of two additional planets orbiting Upsilon Andromedae in the Pegasus constellation, bringing the total count of that star up to three planets.
As of March 2023, we have over 5,300 confirmed discoveries (crossing that historic 5,000 mark in March of last year), and over 9,200 candidates awaiting confirmation for a total of almost 4,000 individual planetary systems. Despite the amazing accomplishments in engineering, technology, optics, and more, detecting larger exoplanets is still the most common as they have the biggest impact on a planet either in terms of light or gravity, but this is improving!
Of the confirmed exoplanets, over 1,800 are Neptune-like, over 1,600 are gas giants, over 1,600 are “Super Earths”, almost 200 are terrestrial, and there is a handful that have not yet been classified.
Almost 4,000 were found using the transit method (74.9%), and just over a thousand were found with radial velocity (19.4%). About 4.5% have been found using microlensing or direct imaging. 6 have been found with pulsar detection and two have been found using astrometry which measures the tiny movements of the star in relation to other stars. The majority of discoveries have an orbit period between 10 and 1,000 Earth days and a mass between a couple of times the size of Earth to over a thousand times the mass of Earth.
The public access to this data is truly remarkable, especially through NASA’s Exoplanet Exploration portal which provides amazing data, graphics, various visualizations, and various fun content to help you explore this data even if you are not an astronomer. The Discoveries Dashboard provided much of the above data quickly and efficiently so that you don’t have to analyze the entire exoplanet catalog unless you want to (which provides much more extensive information and the ability to work with the data yourself). If you are interested in exploring all the possibilities that this information means in fun and artistic ways, I highly recommend the Exoplanet Travel Bureau which combines the work of NASA scientists, futurists, and artists to imagine exoplanet tourism with guided tours of the surfaces of planets, travel posters, introductions to the telescopes exploring them, and more. In addition, NASA has created fantastic simulations using real data to help you fly through space and explore different discovered star systems with NASA Eyes on Exoplanets.
With data and tools like these, it’s fascinating to imagine the possibilities that the existence of these exoplanets implies no matter your age. While we know that our solar system is not unique in having planets, there are still many differences with these other star systems. How would our planet be different if we were orbiting a red or blue star instead of a yellow one? Or if we were part of a binary star system? What if we were closer or farther away? What if our atmosphere contained high levels of helium?
Understanding our similarities helps us understand our differences and vice versa. It helps us understand how we are just one small part of the universe, one of the likely hundreds of billions of planets within our galaxy, which is also one of the trillions of galaxies in our universe. This does not discount our little planet or our lives (though it can be comforting to remember how small we are when things don’t go as well as we hope), but actually reminds us that we are part of the universe and still beautiful and fascinating.
As we continue to explore the universe and uncover data relating to just how well our little area of space lines up with other neighborhoods in space, we better understand… well, everything. From ancient ideas of being the center of the solar system and the universe to discovering that while we are special, there are other planets in our solar system, we better understood how our planet and our neighborhood in space came to be. We also were able to agree on the definition of a planet, which helps us better classify our system as well as others. Like other revolutions in science, future data may adapt this definition once again, but for now, it helps us classify like bodies in astronomical systems to the best of our ability.
Extending our methods of exploring the solar system helped us discover planets that orbit stars other than our own, which we dubbed extrasolar planets, or exoplanets for short. Utilizing the radial velocity and transit methods in addition to newer developing methods such as gravitational microlensing and direct imaging, the discoveries and confirmations continue to pour in at ever-increasing speeds. We know that planets are common among stars and we are slowly but surely uncovering an array of sizes of planets and terrestrial planets in addition to the bigger and more-easily-found gas giants, revealing more that could potentially be like Earth in addition to the ones so different than us. Uncovering data about these exoplanets such as the content of their atmosphere helps us imagine new possibilities of the environments on these worlds, including the possibility of life both similar to and drastically different than our own.
The current results boggle the mind and the developing technologies that will help us to see better and further will only continue to open doors into other systems in space, helping us understand each and our universe as a whole a little better.
Sarah Hoffschwelle is a freelance writer who covers a combination of topics including astronomy, general science and STEM, self-development, art, and societal commentary. In the past, Sarah worked in educational nonprofits providing free-choice learning experiences for audiences ages 2-99. As a lifelong space nerd, she loves sharing the universe with others through her words. She currently writes on Medium at https://medium.com/@sarah-marie and authors self-help and children’s books.