If you look up at the sky on a clear night, you may be able to spot some bright dots which, unlike stars, do not twinkle. These are planets; Mercury, Venus, Mars, Jupiter, and Saturn are the five brightest planets in the night sky. To observe the other two planets of our solar system, Neptune and Uranus, we need telescopes.
All these eight planets rotate around our Sun, which is considered a relatively average star. But wait! Even with our naked eye we can see that there are thousands and thousands of other stars in the sky. In fact, careful estimates show there are over a 100 billion stars in our Milky Way galaxy alone. Do these stars also have planets rotating around them? This was an age old question that kept astronomers up for ages. Although it seems like a reasonable proposition that there are planets around other stars, planets are extremely small and faint objects, making them very hard to observe. In the absence of any proof it was more in the realm of science fiction than science. The conclusive proof of the concept came only recently, in the early 1990s.
The story of the discovery of first exoplanets—planets that go around other stars —is quite fascinating. In 1992, two astronomers, Aleksander Wolszczan and Dale Frail, were looking at the sky using a radio telescope (1). They weren’t looking for planets but studying the remains of a supernova — a pulsar named PSR B1257+12 or Lich. Pulsars are dense, spinning remnants of massive stars that emit beams of radiation or pulses of light, much like a lighthouse does at night. Pulsars rotate very fast (some rotate about once per second, some rotate more than a thousand times per second) and more importantly, they are very punctual! Light signals from a pulsar come at very regular intervals. However, light signals from this particular pulsar were slightly irregular. They realized that the wobble was being caused by two planets that were rotating around it. Thus, the first exoplanets were discovered, though their environment, bathed in radiation from the pulsar, was nothing like Earth.
A pulsar (red sphere) is emitting beams of light. The pulsar rotates fast, and during the rotation if the light beam reaches Earth, we can observe the pulsar. The light pulses are emitted in extremely regular intervals. However, in 1992, scientists found a pulsar whose pulses showed some periodic irregular behaviour. After a thorough investigation, they concluded that two planets are rotating around this pulsar, making it wobble and causing the irregularity. Thus the first exoplanets were discovered.
Three years later, in 1995, Swiss astronomers Michel Mayor and Didier Queloz discovered 51 Pegasi b, the first exoplanet orbiting a Sun-like star. This planet was called “hot Jupiter,” a hot gas giant so close to its star that its year lasted just four days. They received the 2019 Nobel Prize in Physics for this discovery. To date (9th January, 2025), scientists have discovered 5,811 exoplanets (2) using advanced instrumentation. Exoplanets are firmly in the arena of science now.
How do scientists actually detect them? There are currently four main ways to find these planets.
1. Transit method – During the course of a planet’s rotation around its star, if the planet comes between the star and an observer on Earth, a small fraction of the starlight gets blocked by the planet. By observing the amount of light that comes from this star, scientists can detect a small dimming effect and from this dip, they can deduce the existence of this planet.
The curve above shows the brightness of the star observed by scientists at different times. Astronomers call it a light curve. This is the same phenomenon that happens during a solar eclipse, when the moon comes between us and the Sun and blocks sunlight temporarily. The amount of dimming and length of the planet transit we see in the light curve can tell us about the size of the planet and its distance from the star. This is the most popular method to find exoplanets, and so far about 4300 planets2 have been discovered this way (3, 4).
The transit method of detecting exoplanets. The light curve shows the amount of starlight that observers see from Earth over time. When the planet comes between us and the star, a fraction of the starlight is blocked and by observing this dip in the star’s brightness, the planet can be detected.
2. Radial velocity method – In a star-planet system, the star and its planet exert gravitational force on each other. The star, being much more massive than the planet, wins this tug-of-war and the planet revolves around it. However, the planet’s gravitational pull also causes the star to revolve in a very small circle or to wobble. The bigger the planet is, the more it pulls the star, making the movement of the star more pronounced. For example our Sun moves with a speed of 13 m/s due to Jupiter’s pull, but only 9 cm/s due to Earth’s pull. This wobble of distant stars can be measured when scientists observe starlight. When the star moves towards us, it appears a little bluer and when it moves away from us, it appears a little redder. Using this small variation in starlight, sensitive instruments can then deduce that there is a planet around the star and find out its mass and distance. This method was used to discover the first exoplanet around a Sun-like star, 51 Pegasi b. As of 2025, about 1100 planets (2) have been discovered using this method (3, 4).
The radial velocity method of detecting exoplanets. Due to the gravitational pull of the planet, the star also moves a little bit (notice the small circle). During this movement of the star, when it comes toward us, the starlight appears a little bluer and when it moves away from us it appears a little redder. By measuring this difference, the existence of a planet around the star can be detected.
3. Direct imaging – Planets are millions of times fainter compared to their stars, so taking direct pictures of these far-off planets is an extremely difficult prospect. However, in recent years scientists have developed technology that can help block starlight and take a direct image of the planet itself. It is especially effective if the planet is very big, located far away from the star, and hot—such planets glow brightly in infrared light. To date, 82 planets (2) have been discovered by direct imaging (3, 4, 5). 
Direct imaging method of detecting exoplanets. This image shows light from three planets orbiting their star named HR8799, which is marked with the ‘x’. This planet system is 120 light years away from us.
4. Gravitational microlensing – General relativity has shown us that massive objects warp the space around them, and when light passes close to these objects, the warped space causes the light to bend towards them. Thus a massive object can bend light around it and behave like a lens to the observer.
If a star-planet system passes between us and a distant source star, light from the distant star can be lensed by both the star and the exoplanet. By precisely measuring the change in the source star’s brightness due to the lensing effect, scientists can infer the existence of the exoplanet. Scientists have found 232 planets (2) this way (3, 4).
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Gravitational microlensing method of detecting exoplanets. When a star-planet system comes between us and a distant source star, their gravity can act as a lens and make the distant star appear brighter temporarily. By measuring this change, the exoplanet can be detected.
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There are some other methods that have been utilized to discover some exoplanets, for example astrometry, eclipse time variations and pulsar time variations. However these methods are more difficult to employ and have not led to the discovery of many exoplanets yet (2,4).
Although the number of detected exoplanets are increasing every day, so far we have only explored the immediate neighbourhood around our Sun. There are still countless stars and their planetary systems in our galaxy and beyond, waiting to be discovered. However, finding these planets is only the first step. In the next stage, we will search for signs of life in these distant worlds; the research on this has only just begun. Then finally, we may be able to answer that eternal question: Are we alone in the universe?
References:
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- https://ui.adsabs.harvard.edu/abs/1992Natur.355..145W/abstract
- The number of exoplanets for various methods are taken from https://exoplanetarchive.ipac.caltech.edu/docs/counts_detail.html
- https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/
- https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets
- https://www.universetoday.com/140341/what-is-direct-imaging/
- https://astrobites.org/2023/01/02/pulsar-planets/
Author-
Dr. Moupiya Maji is a postdoctoral researcher at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, currently specializing in astronomy education and its role in enhancing science learning. She holds a Ph.D. in computational astrophysics from The Pennsylvania State University and has previously worked at the University of Geneva. Her astrophysics research focused on simulations of stars, galaxies and the early universe. Actively involved in science outreach, she delivers lectures, creates educational content, and conducts training programs for teachers and students.
Editors-
Ananya Sen and Roopsha Sengupta
Image Credits-
