More often than not, astronomers have a specific something they’re looking for when searching the cosmos. But the universe is achingly huge and mysterious, leading to discoveries no one ever set out to find.
These unexpected catches often end up being way cooler and more significant than what astronomers intended to explore. Here are ten of our favorite “accidental” cosmic discoveries—unintentional findings that nevertheless contributed greatly to our understanding of the universe.
1. Uranus (1781) 
An infrared composite image of the two hemispheres of Uranus obtained with Keck Telescope adaptive optics. Credit: JPL/Lawrence Sromovsky (University of Wisconsin-Madison)/W.W. Keck Observatory
In the spring of 1781, British astronomer William Herschel found a faint, sluggish object in the constellation Gemini. At first, Herschel, who was cataloguing stars at the time, was convinced that the object was a comet. Follow-up observations revealed that it had moved across the sky, and apparent comet-like features were visible. Later, Finnish-Swedish astronomer Anders Johan calculated the orbit of Herschel’s discovery, which strongly suggested that this was a planet, later named Uranus, and not a comet.
2. Ceres, the first asteroid…uh, dwarf planet (1801) 
An image of Ceres, produced by the German Aerospace Center in Berlin, combines images taken during Dawn’s first science orbit in 2015 using the framing camera’s red, green, and blue spectral filters. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Similarly, Italian astronomer Giuseppe Piazzi was trying to create an accurate map of star positions when he noticed a strange outlier “star” that kept moving across the sky. Piazzi also thought he was looking at a comet, but subsequent observations hinted that the object was a new planet orbiting the space between Mars and Jupiter.
Further analysis stripped Ceres of its planetary status, and for a long time, it was considered the first asteroid ever discovered. Then, during the great purge of Pluto in 2006, Ceres was reclassified as a dwarf planet.
3. Solar flares (1859) 
An X-class solar flare erupted on the left side of the sun on the evening of Feb. 24, 2014. Credit: NASA/SDO
In 1859, British astronomer Richard Carrington inadvertently documented what would become known as the Carrington Event. He was studying sunspots at the time and had his telescope pointed at our host star when he witnessed a sudden, intense flash of light, later identified as a solar flare. The flare led to the strongest geomagnetic storm ever detected on Earth and the discovery of an entirely new stellar phenomenon.
4. Cosmic X-rays (1962) 
A composite image showing the stellar cluster NGC 1333. The X-ray signals from NASA’s Chandra X-ray Observatory are shown in pink. Credit: NASA/CXC/JPL-Caltech/NOAO/DSS
If this list is any guide, the mid-20th century was a particularly fruitful time for astronomy. One important finding from this period is that the Sun radiates X-rays. A team led by Italian-American astrophysicist Riccardo Giacconi sought to learn if solar X-rays bounced off the Moon and created lunar X-rays.
Instead, they found something much bigger—evidence of an X-ray background originating from outside the solar system. Their finding informed the development of numerous X-ray telescopes, which have been instrumental in shedding light on a variety of cosmic mysteries.
5. The cosmic microwave background (1964) 
This map shows the oldest light in our universe, as detected with the greatest precision yet by the ESA Planck mission. Credit: JPL/ESA/Planck Collaboration
In May 1964, Arno Penzias and Robert Wilson were testing how radio waves bounced off balloon satellites developed by Bell Telephone Laboratories. But they kept getting an unpleasant hissing noise, in addition to an unexplained heat signal. Even after eliminating disturbances—including a particularly persistent flock of pigeons—the noise persisted.
“And we, of course, were worried—‘What’s wrong with this system?’” Wilson told the New York Times in an earlier interview. “We were at wit’s end.”
Fortunately, the fault was merely in the stars. The pair had stumbled upon evidence of the cosmic microwave background, a “relic” of the explosive birth of our universe—the Big Bang.
6. Pulsars (1967) 
A close-up of the Crab Nebula showing the central neutron star, whose radiation signals alerted Bell and her colleagues to the first identified pulsars. Credit: NASA/ESA/J. Hester (ASU)/M. Weisskopf (NASA/MSFC)
Northern Irish physicist Jocelyn Bell detected a bit of “scruff” in the data recorded by a radio telescope she helped build. Bell, a graduate student at the time, paid no heed to doubts from her colleagues and continued to study the strange pulsation for the next three months. Her tenacity paid off; Bell confirmed that the weird light was a pulsating signal from afar—the first known pulsar, which was later identified to be a rotating neutron star.
This discovery earned Sir Martin Ryle and Antony Hewish the 1974 Nobel Prize in Physics, although the Nobel committee neglected to recognize Bell’s critical contributions to the finding.
7. Gamma-ray bursts (1967) 
Rings of dust spewed out by the brightest gamma ray burst ever found. The observation was made by the XMM-Newton Observatory. Credit: ESA/XMM – Newton/M. Rigoselli (INAF)
Gamma-ray bursts (GRBs) caught the attention of U.S. satellites on the lookout for nuclear attacks during the Cold War. Defense satellites detected around 15 instances of strange gamma-ray signals too weird to come from nuclear tests. Finally, Los Alamos National Laboratory stepped in to investigate, and in 1973 the astronomical community was alerted to the existence of gamma-ray bursts—the most powerful source of energy in the universe.
To say GRBs caused a big splash would be a wild understatement. Astronomers suddenly had a new cosmic source to explain countless previously unidentified light signals. To put this into perspective, a literature review found that between 1973 and 2001, around 5,300 papers were published on GRBs.
8. The first exoplanet (1992) 
An artist’s impression of globular cluster M4, where astronomers discovered PSR B1620-26 b, the first exoplanet to be identified and confirmed. Credit: NASA/G. Bacon (STScI)
Astronomers had long believed in the existence of exoplanets—planets orbiting stars that are not our Sun—but it took centuries of false alarms and controversy before scientists found something that was indisputably an exoplanet. While studying a pulsar, astronomers Aleksander Wolszczan and Dale Frail spotted a pair of planets—yep, two at the same time—orbiting a neutron star.
Equipped with more sophisticated instruments, astronomers are now finding exoplanets at a steady clip. Just a couple weeks ago, NASA’s official exoplanet repository reached 6,000 exoplanets.
9. Evidence for dark energy (1998) 
An artist’s impression of the early universe. Credit: NASA/MSFC
Until 1998, astronomers generally believed that, although the universe’s expansion accelerated after the Big Bang, gravity would eventually slow it down. Then, two separate teams of astronomers observed an unusually dim Type 1a supernova. After studying its distance and spectra, cosmologists realized that the universe is expanding at an accelerating rate, rather than slowing down as expected. To make sense of this observation, they proposed the existence of a hypothetical force: dark energy. If dark matter adds to the universe’s mass, pulling things together, dark energy does the opposite—driving matter apart and accelerating the universe’s expansion.
10. Fast radio bursts (2007) 
An artist’s impression of a magnetar losing material into space, which may have caused a fast radio burst detected by NASA in 2022. Credit: NASA/JPL-Caltech
In accidental astronomy, one accident seems to lead to another. While parsing through pulsar data. In 2007, astrophysicist Duncan Lorimer and his then-graduate student David Narkevic found a 2001 record of an extremely short radio burst—lasting just 5 milliseconds—that released an entire month’s worth of the Sun’s energy.
“There aren’t too many things in the universe that can do that,” Lorimer told New Scientist at the time. Pulsars emit radiation at consistent intervals, so fast radio bursts must have come from single, cataclysmic events—at least, that’s what scientists believe. This discovery is so recent that many mysteries still surround fast radio bursts.