Astronomical transit facts for kids
In astronomy, a transit is when one space object passes directly in front of a larger one. This happens as seen from a specific viewpoint, like Earth. The object in front appears to move across the face of the larger object, covering a small part of it.
The word "transit" is used when the object passing in front looks smaller than the one behind it. If the front object looks bigger and completely hides the one behind it, that's called an occultation.
Seeing a transit is rare because the objects need to line up almost perfectly. But when they do, scientists can learn a lot about the planet and its star!
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Transits in Our Solar System
Sometimes, a planet passes right in front of the Sun as seen from Earth. This only happens with planets closer to the Sun than Earth. These are inferior planets, like Mercury and Venus. We call these events transits of Mercury and transits of Venus.
What you see depends on where you are! For example, if you were on Mars, you could see Earth transit the Sun. Even our own Moon can transit the Sun if you're far enough away, like a spacecraft. From far away, the Moon looks much smaller than it does from Earth.
We also use 'transit' when a moon passes in front of its planet. For example, you can sometimes see Io, Europa, Ganymede, or Callisto – Jupiter's big moons – transit across Jupiter from Earth.
It's super rare, but sometimes four space objects can line up! For instance, on June 27, 1586, people on Venus would have seen Mercury transit the Sun. At the very same moment, people on Saturn would have seen both Mercury and Venus transit the Sun!
Cool Transit Observations
No spacecraft were ready to watch Earth transit the Sun from Mars on May 11, 1984. The next chance to see this will be in 2084!
On December 21, 2012, the Cassini–Huygens spacecraft, orbiting Saturn, saw Venus pass in front of the Sun.
On June 3, 2014, the Curiosity rover on Mars watched Mercury transit the Sun. This was the first time a planet's transit was seen from another planet besides Earth!
Planets Passing Each Other
Very rarely, one planet can even pass in front of another planet! If the planet closer to us looks smaller than the one behind it, we call this a mutual planetary transit.
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Transit of Venus as seen from Earth, 2012
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Mercury transiting the Sun, seen from Curiosity rover on Mars (3 June 2014).
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The Moon transiting in front of Earth, seen by Deep Space Climate Observatory on 4 August 2015.
Finding Planets Around Other Stars
Scientists use the transit method to find exoplanets – planets orbiting stars other than our Sun. When an exoplanet passes in front of its star, it blocks a tiny bit of the star's light. We can measure this dip in light to create a 'light curve'.
This light curve tells us things about the planet and its star, like how big they are. Scientists need to see several transits happening regularly to learn more. If a star has many planets, their gravity can pull on each other, causing slight changes in transit times. This is called Transit-Timing Variation (TTV).
It's not easy to spot a transit from Earth. The chance of seeing one is quite low. The probability depends on the size of the star and planet, and how far the planet is from its star.
This formula helps scientists figure out the chance of seeing a transit. Rstar is the star's size, Rplanet is the planet's size, and a is how far the planet is from its star.
Because transits are rare, telescopes have to watch huge parts of the sky all the time. 'Hot Jupiters' – big planets orbiting very close to their stars – are easier to spot because they block more light. To find Earth-sized planets, scientists look at red dwarf stars, which are much smaller than our Sun. Even with the low chance, transiting is a great way to find new exoplanets!
Finding extrasolar planets (planets outside our Solar System) has made scientists very interested in watching them transit their own stars. HD 209458b was the very first exoplanet found this way!
Watching transits is one of the best ways to study exoplanet systems today. This method, called transit photometry, is how most exoplanets are found! When a planet moves in front of its star, the star's light gets a little dimmer. Bigger planets cause a bigger dip, making them easier to find. Scientists often use other ways to check if it's really a planet.
There are currently (July 2025) 2782 planets confirmed with Kepler light curves for stellar host.
Stages of a Transit
During a transit, there are four special moments called 'contacts'. These are when the edge of the smaller object touches the edge of the larger object. Long ago, measuring these exact times helped scientists figure out where planets and stars were in space. Here's how they happen:
- First contact: The smaller object just begins to touch the edge of the larger object, moving inward.
- Second contact: The smaller object is now fully inside the edge of the larger object, continuing to move across it.
- Third contact: The smaller object reaches the other side and just begins to touch the inner edge of the larger object, moving outward.
- Fourth contact: The smaller object has completely moved off the larger object and is now fully outside it.
There's also a 'greatest transit' point. This is when the centers of the two objects are closest to each other, usually halfway through the transit.
Telescopes That Find Transiting Planets
Because it's a simple way to scan huge parts of space, transit photometry has been the most successful method for finding exoplanets. Many projects use this technique. Some, like HATNet, KELT, Kepler, and WASP, have been very successful. New missions like TESS are also underway. You can find more on the List of Exoplanet Search Projects.
HATNet Project
The HATNet Project uses many small telescopes. Some are in Arizona and Hawaii. Others are spread across Africa, Australia, and South America (called HATSouth). These telescopes can see a wide area of the sky. Because they are all over the world, they can watch the sky 24/7. This helps them catch many transiting planets, especially those with short orbits.
A new part of the project, HATPI, is being built in Chile. It will scan most of the night sky from there.
KELT Telescopes
The KELT project uses telescopes on Earth to find transiting planets. It started in October 2004, with a second telescope added in 2009. KELT North watches a wide strip of sky over North America. KELT South looks at specific areas. Both telescopes are so sensitive they can spot even a tiny 1% dip in a star's light. This helps them find planets similar to those in our own Solar System!
Kepler Space Telescope
The Kepler space telescope was a very important mission. From 2009 to 2013, it stared at one part of the sky, looking for transiting planets in constellations like Cygnus, Lyra, and Draco. After 2013, even with some equipment issues, Kepler kept working until 2018. It then changed its view every 75 days to look at different parts of the sky.
TESS Mission
The TESS was launched on April 18, 2018. It's designed to scan almost the entire sky! TESS looks at different sections of the sky for about 27 days each. Some areas, especially near TESS's spin axis, are watched for up to a year. This helps TESS find planets that take a long time to orbit their stars.
See also
- Eclipse
- Kepler Mission
- Occultation
- Syzygy (astronomy)
- Transit of asteroids
- Transit of Deimos from Mars
- Transit of Phobos from Mars
- Transit of Vulcan
- Transit of Mercury from Mars
- Transit of Earth from Mars
