Reference Terms
from Wikipedia, the free encyclopedia
Definition of planet
The definition of "planet" has for some time been the subject of intense debate. Although the word dates back thousands of years, no officially decreed scientific definition of "planet" existed before the early 21st century. Until the beginning of the 1990s, there was little need for one, as astronomers had only a single sample of planets in solar system to study, and one small enough for its many irregularities to be dealt with individually.
However, after 1992 and the discovery of the myriad tiny worlds beyond the orbit of Neptune, the size of the sample rose from nine to at least several dozen. Following the discovery of the first extrasolar planet beyond our solar system in 1992, the number of samples has now reached hundreds. These revelations not only increased the number of potential planets, but, in their variety and peculiarity (some nearly large enough to be stars, others smaller than our Moon) challenged long perceived notions of what a planet could be.
Related Stories
1
2
Space & Time News
April 26, 2026
Apr. 12, 2026 A major international effort has produced an ultra-precise measurement of the Universe’s expansion rate, confirming it’s faster than early-Universe models predict. By linking multiple distance-measuring techniques, scientists ruled out simple ...
Apr. 10, 2026 Scientists have proposed a surprising new way to detect gravitational waves—by observing how they change the light emitted by atoms. These waves can subtly shift photon frequencies in different directions, leaving behind a detectable signature. ...
Apr. 9, 2026 Earth’s nights are steadily getting brighter overall, but the changes vary dramatically by region. Rapid urban growth is lighting up countries like China and India, while parts of Europe are dimming due to energy-saving efforts and new lighting ...
Apr. 8, 2026 Planetary exploration may be about to get a major speed boost. Researchers tested a semi-autonomous robot that can move from rock to rock, analyzing each without waiting for human instructions. The system completed missions up to three times faster ...
Apr. 3, 2026 A new concept suggests SpaceX’s Starship could revolutionize a future mission to Uranus, one of the solar system’s most overlooked planets. By refueling in orbit and helping slow the spacecraft on arrival, it could cut travel time nearly in ...
Mar. 20, 2026 A mysterious spike of platinum buried deep in Greenland’s ice has long fueled theories of a catastrophic comet or asteroid strike 12,800 years ago—possibly triggering a sudden return to icy conditions known as the Younger Dryas. But new research ...
Mar. 14, 2026 Mars didn’t always look like the barren world we see today. Over billions of years, the Sun’s solar wind stripped away much of its atmosphere, helping transform it from a warmer, wetter planet ...
Feb. 24, 2026 Scientists have proposed a surprising connection between solar flares and earthquakes. When solar activity disturbs the ionosphere, it may generate electric fields that penetrate fragile fracture zones in Earth’s crust. If a fault is already ...
Feb. 2, 2026 NASA’s Perseverance rover has just made history by driving across Mars using routes planned by artificial intelligence instead of human operators. A vision-capable AI analyzed the same images and terrain data normally used by rover planners, ...
Feb. 2, 2026 Hidden lava tunnels on the Moon and Mars could one day shelter human explorers, offering natural protection from radiation and space debris. A European research team has unveiled a bold new mission concept that uses three different robots working ...
Jan. 28, 2026 Low-Earth orbit is more crowded—and fragile—than it looks. Satellites constantly weave past each other, burning fuel and making dozens of evasive maneuvers every year just to stay safe. A major solar storm could disable navigation and ...
Jan. 19, 2026 Physicists have unveiled a new way to simulate a mysterious form of dark matter that can collide with itself but not with normal matter. This self-interacting dark matter may trigger a dramatic collapse inside dark matter halos, heating and ...
Latest Headlines
updated 12:56 pm ET
Apr. 25, 2026 A major physics experiment has uncovered evidence for a strange new form of matter, where a fleeting particle gets trapped inside a nucleus. This exotic state may reveal how mass is generated, ...
Apr. 25, 2026 In the chaotic first moments after the Big Bang, ripples in spacetime may have done more than just echo through the cosmos—they could have helped create dark matter itself. New research suggests ...
Apr. 24, 2026 Scientists are grappling with a cosmic mystery: why does the Universe behave differently on massive scales compared to our own solar system? While ...
Apr. 24, 2026 A mysterious cosmic explosion has astronomers buzzing, as a strange event may hint at an entirely new kind of stellar cataclysm. After detecting ripples in space-time, scientists spotted a ...
Apr. 22, 2026 Scientists have discovered unexpected water-ice clouds on a distant, Jupiter-like exoplanet, challenging current atmospheric models. By directly imaging Epsilon Indi Ab with the James Webb Space ...
Apr. 21, 2026 Deep inside planets like Uranus and Neptune, scientists may have uncovered a bizarre new state of matter where atoms behave in unexpected ways. Advanced simulations suggest that carbon and hydrogen, ...
Apr. 19, 2026 Astronomers have long been puzzled by a cosmic mystery: planets orbiting two stars—like Star Wars’ Tatooine—are surprisingly rare, even though they should be common. New research suggests the ...
Apr. 18, 2026 Scientists have captured stunning new insights into one of the universe’s most powerful phenomena—black hole jets—by using a planet-sized network of radio telescopes. Focusing on Cygnus X-1, ...
Apr. 18, 2026 As the Moon swallowed the Sun during the April 8, 2024, total solar eclipse, something remarkable happened on the ground—cities went eerily quiet. Scientists analyzing seismic data found that ...
Apr. 15, 2026 A new study proposes detecting life in space by spotting patterns across many planets instead of focusing on one at a time. If life spreads and changes planetary environments, it could leave behind ...