By Ali Hamza 
For most of human history, we only knew about the planets in our own solar system. These were the wandering stars our ancestors could see with their own eyes, and later, the distant giants revealed by the first telescopes. The idea of planets orbiting other suns was just a dream, a story told in science fiction. It seemed impossible to find something so small, so dark, and so incredibly far away.
But today, astronomers announce the discovery of new planets almost every week. We now know of thousands of worlds beyond our solar system, called exoplanets. They are strange and diverse, with conditions unlike anything we have at home. Some are scorching gas giants orbiting dangerously close to their stars, while others might be rocky worlds floating in the eternal twilight of deep space.
This leads to a fascinating puzzle. If these planets are light-years away and often hidden by the blinding glare of their own suns, how can we possibly know they are there? We can’t take a picture with an ordinary camera. The methods scientists use are like being a master detective, looking for tiny clues and subtle hints that betray the presence of a hidden world. So, how do you find a planet you cannot see?
The secret is that a planet may be invisible, but its gravity is not. Imagine a giant parent spinning a small child around in a circle. The parent is much bigger and stronger, so the child does most of the circling. But if you look closely, you will see the parent is not standing completely still. They are wobbling just a little bit on the spot. The child’s tiny tug causes the parent to move in a very small circle of their own.
This is exactly how the “wobble” method works in space. A star, with its immense gravity, is the parent. The planet, even a large one, is the child. As the planet orbits the star, its own gravitational pull gives the star a tiny, gentle tug. This causes the star to wobble in a small circle or ellipse. From our viewpoint on Earth, the star appears to move slightly towards us and then away from us in a regular, repeating pattern.
Scientists can detect this wobble by studying the star’s light. When a star moves towards us, its light waves get squeezed together, making the light appear slightly bluer. When it moves away, the light waves are stretched out, making it appear slightly redder. This is called the Doppler effect, similar to how a siren’s pitch changes as it races past you. By measuring these delicate color shifts in the star’s light with incredibly precise instruments, astronomers can deduce that an invisible planet is there, tugging on its sun. They can even figure out the planet’s mass and how long its year is based on the rhythm of the wobble.
Another brilliant way to find planets is by watching for a brief shadow. Think about a moth flying in front of a bright streetlamp. For a moment, the light dims just a little before returning to normal. On a cosmic scale, this is what happens during a “transit.” When a distant planet’s orbit is lined up just right from our perspective, it will pass directly in front of its host star.
This passage, or transit, blocks a tiny fraction of the star’s light. It is not a total blackout, but a very small, very specific dip in brightness. Our powerful telescopes, like NASA’s Kepler and TESS missions, are designed to constantly monitor the brightness of thousands of stars, watching for these tell-tale flickers. When they see a star dim by a small amount for a few hours and then brighten again, and when this happens like clockwork every orbit, it is strong evidence of a planet.
The transit method tells us a lot more than just the planet’s existence. The depth of the dimming tells us how big the planet is. A giant planet blocks more light than a small, rocky one. The time between dips tells us the length of the planet’s year. Even more amazingly, when the starlight passes through the planet’s atmosphere during the transit, scientists can analyze that light to figure out what gases make up the atmosphere. They can search for water vapor, methane, or other signs of interesting chemistry, all from a tiny flicker of light years away.
Taking a direct picture of an exoplanet, what astronomers call “direct imaging,” is the hardest method of all, like trying to spot a firefly sitting right on the edge of a giant, brilliant searchlight. The star is millions of times brighter than the planet, completely overwhelming its faint glow. So how do scientists even attempt it?
They use incredibly clever technology to block out the star’s blinding light. Telescopes are equipped with a small, black disk called a coronagraph, which acts like an artificial eclipse, physically blocking the central star inside the telescope. This allows the much fainter light from the orbiting planet to be seen. Another technique involves using a special mask to cancel out the starlight through wave interference.
Even with these tools, it is a monumental challenge. It only works for planets that are very large, very young and hot (so they are still glowing with heat from their formation), and orbiting very far from their star. The planets we have imaged so far look like tiny, lonely dots of light. But from that single dot, we can learn about the planet’s temperature, and in some cases, what its atmosphere might be made of. Each direct image is a monumental achievement, a real portrait of a new world.
One of the most bizarre and fascinating methods relies on a prediction from Einstein’s theory of relativity. Einstein showed that massive objects, like stars, warp the fabric of space and time around them. Light, traveling in a straight line, will bend its path when it passes near this warped space. This means a massive object can act like a lens, bending and focusing light from a more distant object behind it.
This effect is called gravitational microlensing. Here is how it works for finding planets. Sometimes, a star from our galaxy will drift in front of a much more distant, background star. The closer star’s gravity acts as a lens, magnifying the light from the background star for a few weeks or months, causing it to temporarily brighten. If the closer star has a planet orbiting it, the planet’s own gravity adds a tiny extra bump to the lensing effect. It creates a short, sharp spike in the brightness of the background star.
This spike is the signature of the planet. This method is incredibly sensitive and can find planets that are very far away, even in the central bulge of our galaxy. It is a chance event, as it requires a perfect alignment of two stars and a planet, but it allows us to discover worlds that are otherwise completely undetectable, expanding our search to the most remote corners of the Milky Way.
The wobble method measures a star’s movement towards and away from us. But what about its side-to-side movement in the sky? This is the goal of “astrometry.” This technique involves making the most precise measurements possible of a star’s exact position in the sky. If a star is being tugged by an orbiting planet, it will not just wobble back and forth in place; it will also trace a very small, circular path in the heavens.
Imagine trying to measure the width of a coin from several miles away. That is the level of precision required for astrometry. For a long time, our technology was not good enough to see this tiny motion. However, with space telescopes like Gaia, which is mapping the positions and motions of over a billion stars with unbelievable accuracy, this method is becoming more powerful. By tracking a star’s path over years, scientists can detect the minuscule sway caused by the gravitational pull of an unseen planet, giving them another way to confirm its presence and measure its mass.
Every new planet discovered is a new piece in the greatest puzzle of all: are we alone? By finding thousands of exoplanets, we are learning that our solar system is just one of many possible configurations. Some planetary systems look nothing like our own, which challenges our old ideas about how planets form. We are learning what kinds of worlds are common and what kinds are rare.
The ultimate goal, of course, is to find a world like Earth. A rocky planet, orbiting at just the right distance from its star for liquid water to exist—a region astronomers call the “habitable zone.” By studying the atmospheres of these planets with future telescopes, we will one day be able to search for “biosignatures,” which are signs of life, like the presence of oxygen and methane in combination. Finding a planet that might support life would change our understanding of our place in the universe forever. Every distant, unseen world we detect brings us one step closer to answering the ancient question of whether life exists beyond Earth.
The hunt for invisible planets is a story of human ingenuity. We cannot travel to these distant suns, but our minds have figured out how to sense their silent companions. We use the subtle pull of gravity, the faint dimming of a light, and even the bending of space itself to reveal worlds we will never see with our own eyes. It is a reminder that the universe is full of wonders, waiting to be discovered not by sight, but by the power of a curious mind.
Do you think the first sign of life on another planet will be discovered by one of these indirect methods, or will we need to wait until we can build a telescope powerful enough to see it directly?
How many exoplanets have been discovered so far?
Scientists have confirmed the discovery of over five thousand exoplanets, and thousands more are considered candidates waiting for confirmation. New planets are being found all the time as our technology and methods improve.
What is the most common type of planet found?
The most common types of planets we have found are ones that don’t exist in our own solar system, called “Super-Earths” and “Mini-Neptunes.” These are planets larger than Earth but smaller than Neptune, representing a size category that is very common in the galaxy but absent from our local planetary family.
Can we see exoplanets with a backyard telescope?
No, it is impossible to see an exoplanet directly with a backyard telescope. The planets are too faint and too close to the blinding glare of their host stars. Discovering them requires the extreme precision of large professional telescopes in space and on the ground.
What is the closest known exoplanet to Earth?
The closest known exoplanet is Proxima Centauri b, which orbits Proxima Centauri, the closest star to our Sun. It is located just over 4 light-years away and is a rocky planet within its star’s habitable zone.
How long does it take to confirm an exoplanet discovery?
It can take months or even years to confirm a planet. Scientists need to observe multiple transits or collect enough wobble data to be sure the signal is from a planet and not some other astronomical phenomenon or instrument error.
What is the habitable zone?
The habitable zone, often called the “Goldilocks zone,” is the region around a star where it is not too hot and not too cold for liquid water to potentially exist on the surface of a rocky planet. This is considered a key ingredient for life as we know it.
Have we found any Earth-like planets?
We have found several rocky exoplanets that are similar in size to Earth and are located in their star’s habitable zone, like those in the TRAPPIST-1 system. However, “Earth-like” is a broad term, and we do not yet know if these planets have atmospheres, water, or conditions that could truly support life.
Which method has found the most exoplanets?
The transit method is responsible for discovering the vast majority of known exoplanets. This is largely thanks to the Kepler Space Telescope, which used this method to find thousands of planets during its mission.
What is a “Hot Jupiter”?
A “Hot Jupiter” is a class of exoplanet that is a gas giant, like our Jupiter, but orbits extremely close to its parent star. This results in very short orbital periods of just a few days and incredibly high surface temperatures.
Could we ever send a probe to an exoplanet?
With our current technology, it is not feasible. The distances are too vast. For example, traveling to the closest exoplanet, Proxima Centauri b, would take tens of thousands of years with our fastest existing spacecraft. Concepts like light sails are being studied, but such a journey remains a dream for the distant future.