The Oort cloud (/ ɔːr t, ʊər t /), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years). It is divided into two regions: a disc-shaped inner Oort cloud (or Hills cloud) …

In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them (special relativistic "kinetic" time dilation) or to a difference in gravitational potential between their locations (general relativistic gravitational time dilation).When unspecified, "time dilation" usually refers to the effect due to ...

Events with a duration of less than about two seconds are classified as short gamma-ray bursts. These account for about 30% of gamma-ray bursts, but until 2005, no afterglow had been successfully detected from any short event and little was known about their origins. Since then, several dozen short gamma-ray burst afterglows have been detected and localized, several of …

The acronyms listed below were placed into one or more of these categories: Astrophysics terminology – physics-related acronyms Catalog – collections of tabulated scientific data; Communications network – any network that functions primarily to communicate with spacecraft rather than performing astronomy Data – astrophysical data not associated with any single …

Nemesis is a hypothetical red dwarf or brown dwarf, originally postulated in 1984 to be orbiting the Sun at a distance of about 95,000 AU (1.5 light-years), somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in the geological record, which seem to occur more often at intervals of 26 million years. [citation needed] In a 2017 paper, Sarah Sadavoy and …

Tyche / ˈ t aɪ k i / is a hypothetical gas giant located in the Solar System's Oort cloud, first proposed in 1999 by astrophysicists John Matese, Patrick Whitman and Daniel Whitmire of the University of Louisiana at Lafayette. They argued that evidence of Tyche's existence could be seen in a supposed bias in the points of origin for long-period comets.More recently, Matese …

Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the peculiar clustering of orbits for a group of extreme trans-Neptunian objects (ETNOs), bodies beyond Neptune that orbit the Sun at distances averaging more than 250 times that of the Earth. These ETNOs tend to make their closest approaches to the Sun in one …

William of Ockham, OFM (/ ˈ ɒ k əm /; also Occam, from Latin: Gulielmus Occamus; c. 1287 – 10 April 1347) was an English Franciscan friar, scholastic philosopher, apologist, and Catholic theologian, who is believed to have been born in Ockham, a small village in Surrey. He is considered to be one of the major figures of medieval thought and was at the centre of the …

Time travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically with the use of a hypothetical device known as a time machine.Time travel is a widely recognized concept in philosophy and fiction, particularly science fiction.The idea of a time machine was popularized …

Zecharia Sitchin (July 11, 1920 – October 9, 2010) was an author of a number of books proposing an explanation for human origins involving ancient astronauts.Sitchin attributed the creation of the ancient Sumerian culture to the Anunnaki, which he stated was a race of extraterrestrials from a planet beyond Neptune called Nibiru.He asserted that Sumerian …