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
January 11, 2026
Jan. 8, 2026 A team of physicists has discovered a surprisingly simple way to build nuclear clocks using tiny amounts of rare thorium. By electroplating thorium onto steel, they achieved the same results as years of work with delicate crystals — but far more ...
Jan. 8, 2026 Nearly everything in the universe is made of mysterious dark matter and dark energy, yet we can’t see either of them directly. Scientists are developing detectors so sensitive they can spot particle interactions that might occur once in years or ...
Jan. 6, 2026 A distant pulsar’s radio signal flickers as it passes through space, much like stars twinkle in Earth’s atmosphere. By monitoring this effect for 10 months, researchers watched the pattern slowly evolve as gas, Earth, and the pulsar all moved. ...
Jan. 6, 2026 Mars looks familiar from afar, but surviving there means creating a protective oasis in a hostile world. Instead of shipping construction materials from Earth, researchers are exploring how to use Martian soil as the raw ingredient. Two tough ...
Jan. 2, 2026 A physicist has proposed a bold experiment that could allow gravitational waves to be manipulated using laser light. By transferring minute amounts of energy between light and gravity, the ...
Dec. 31, 2025 As we age, our immune system quietly loses its edge, and scientists have uncovered a surprising reason why. A protein called platelet factor 4 naturally declines over time, allowing blood stem cells to multiply too freely and drift toward unhealthy, ...
Dec. 25, 2025 Scientists are digging into the hidden makeup of carbon-rich asteroids to see whether they could one day fuel space exploration—or even be mined for valuable resources. By analyzing rare meteorites ...
Dec. 22, 2025 Astronomers have uncovered a massive hidden planet and a rare “failed star” by combining ultra-precise space data with some of the sharpest ground-based images ever taken. Using the Subaru Telescope in Hawaiʻi, the OASIS survey tracked subtle ...
Dec. 19, 2025 Gravitational waves from black holes may soon reveal where dark matter is hiding. A new model shows how dark matter surrounding massive black holes leaves detectable fingerprints in the waves recorded by future space ...
Dec. 17, 2025 Researchers have shown that quantum signals can be sent from Earth up to satellites, not just down from space as previously believed. This breakthrough could make global quantum networks far more powerful, affordable, and ...
Dec. 6, 2025 SQUIRE aims to detect exotic spin-dependent interactions using quantum sensors deployed in space, where speed and environmental conditions vastly improve sensitivity. Orbiting sensors tap into ...
Dec. 3, 2025 Earth’s orbit is getting crowded with broken satellites and leftover rocket parts. Researchers say the solution is to build spacecraft that can be repaired, reused, or recycled instead of abandoned. They also want new tools to collect old debris ...
Latest Headlines
updated 12:56 pm ET
Jan. 11, 2026 The accelerating expansion of the universe is usually explained by an invisible force known as dark energy. But a new study suggests this mysterious ...
Jan. 10, 2026 Scientists have discovered an enormous stream of super-hot gas erupting from a nearby galaxy, driven by a powerful black hole at its center. The jets ...
Jan. 10, 2026 Crystals hidden in Australia’s oldest rocks have revealed new clues about how Earth and the Moon formed. The study suggests Earth’s continents didn’t begin growing until hundreds of millions of ...
Jan. 10, 2026 Astronomers have uncovered the long-hidden cause behind Betelgeuse’s strange behavior: a small companion star carving a visible wake through the giant’s vast atmosphere. Using nearly eight years ...
Jan. 9, 2026 Hubble has revealed a strange cosmic object called Cloud-9, a dark matter–dominated cloud with no stars at all. Scientists believe it is a “failed galaxy,” a leftover building block from the ...
Jan. 8, 2026 Using NASA’s IXPE, astronomers captured an unprecedented view of a white dwarf star actively feeding on material from a companion. The data revealed giant columns of ultra-hot gas shaped by the ...
Jan. 8, 2026 Einstein’s claim that the speed of light is constant has survived more than a century of scrutiny—but scientists are still daring to test it. Some theories of quantum gravity suggest light might ...
Jan. 7, 2026 Scientists have detected a surprisingly hot galaxy cluster dating back to the universe’s infancy. The cluster formed far earlier and burned far hotter than current models predict. Researchers ...
Jan. 7, 2026 Europa’s buried ocean has made it one of the most exciting places to search for life beyond Earth. However, new calculations suggest its seafloor may be calm, cold, and largely inactive, with ...
Jan. 7, 2026 Around the bright star Fomalhaut, astronomers spotted glowing clouds of debris left behind by colossal collisions between large space rocks. One of these clouds was even mistaken for a planet before ...