An Etymological Dictionary of Astronomy and Astrophysics
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→ Interplanetary dust and other small particles organized into thin, flat rings encircling a planet. The most spectacular planetary rings known are those around → Saturn, but the other three → giant planets of the solar system (→ Jupiter, → Uranus, and → Neptune) have their own ring systems.

→ planetary; → ring; → system.

The branch of astronomy that deals with the science of planets, or planetary systems, and the solar system.

→ planetary; → science.

A system composed of a star and all the celestial bodies bound to it by gravity, especially planets and their natural satellites.

→ planetary; → system.

The passage of an → inferior planet against the disk of the Sun, as viewed from Earth. Mercury and Venus pass in front of the Sun only when they are close to one of their → orbital nodes, at → inferior conjunction. For Mercury this occurs at the beginning of November (the → ascending node) or at the beginning of May (the → descending node), while for Venus it takes place at the beginning of December (the ascending node) or at the beginning of June (the descending node). See also → transit of Mercury, → transit of Venus.

→ planetary; → transit.

A space observatory under development by the → European Space Agency for launch around 2024. Its objective is to detect and characterize → exoplanets by means of their → transit signature in front of a very large sample of → bright stars, and measure the seismic oscillations (→ asteroseismology) of the parent stars orbited by these planets in order to understand the properties of the exoplanetary systems.

→ planetary; → transit; → oscillation; → star.

Any of numerous small solid bodies in a → protoplanetary disk that in some cases clump together to form → planets but in other cases remain relatively small and become → asteroids and → comets. Similarly, → Kuiper Belt Objects are probably the remnants of the planetesimals that formed the planets.

From → planet + (infinit)esimal, → infinitesimal.

Xordesayyâré, from xordé "small, minute; crumbs," from xord "minute, little, small" (from Mid.Pers. xvart, xôrt "small, insignificant;" Av. ādra- "weak, dependent;" Skt. ādhrá- "small, weak, poor," nādh "to be oppressed;" Gk. nothros "sluggish;" PIE base *nhdhro-) + sayyâré, → planet.

An older equivalent for → asteroid.

→ planet + → -oid.

Same as → planetary science.

→ planet; + → -logy.

An evolved star whose → planetary nebula has dissipated.

→ post-; → planetary; → nebula; → star.

A short-lived transition object between the → asymptotic giant branch (AGB) and → planetary nebula phases. See also → post-asymptotic giant branch star (post-AGB).

→ pre-; → planetary; → nebula. The more commonly used term, → protoplanetary nebula, is a misnomer and must be avoided. Indeed → protoplanetary is widely used to refer to disks around → pre-main sequence stars. Since the term → protoplanet is used to denote planets undergoing formation, the use of the term "protoplanetary nebula" to mean a completely different kind of object is an unfortunate choice (Sahai et al. 2005, ApJ 620, 948).

A stage in the formation of a → planet, which comes about from the aggregation of → planetesimals. The protoplanet eventually becomes a planet by → accretion of material from a → protoplanetary disk.

→ proto- + → planet.

Of or relating to a → protoplanet or protoplanets.

→ protoplanet; → -ary.

A → circumstellar disk of gas and dust surrounding a → pre-main sequence star from which planetary systems form. Protoplanetary disks are remnants of → accretion disks which bring forth stars. Typically, their sizes are ~100-500 AU, masses ~10^-2 solar masses, lifetimes ~10⁶-10⁷ years, and accretion rates ~10^-7-10^-8 solar masses per year. According to the standard theory of planet formation, called core accretion, planets come into being by the growth of → dust grains which stick together and produce ever larger bodies, known as → planetesimals. The agglomeration of these planetesimals of 100 to 1000 km in size into rocky Earth-mass planets is the main outcome of this theory. Beyond the → snow line in the disk, if the masses of these cores of rock and ice grow higher than 10 times that of Earth in less than a few million years, gas can rapidly accrete and give rise to giant gaseous planets similar to → Jupiter. If core building goes on too slowly, the disk gas dissipates before the formation of → giant planets can start. Finally the left-over planetesimals that could not agglomerate into rocky planets or core of giant planets remain as a → debris disk around the central object that has become a → main sequence star. An alternative to core accretion theory is formation of planets in a massive protoplanetary disk by → gravitational instabilities. The validity of these two theories is presently debated. See also → protoplanet.

→ protoplanet; → disk.

→ preplanetary nebula.

→ proto-; → planetary; → nebula.

A planet orbiting a → pulsar. The first such planet to be discovered was around a → millisecond pulsar known as PSR 1257+12.

→ pulsar; → planet.

Same as → free-floating object.

→ rogue; → planet.

The → extreme adaptive optics system and → coronagraphic facility at the → European Southern Observatory (ESO) → Very Large Telescope (VLT) (UT3) available from May 2014. Its primary science goal is imaging, low-resolution spectroscopic, and polarimetric characterization of → exoplanetary system at → visible and → near-infrared wavelengths (0.5-2.32 μm). SPHERE is capable of obtaining → diffraction-limited images at 0''.02 to 0''.08 resolution depending on the wavelength. Its → spectral resolution is 30 to 350, depending on the mode.

→ spectro-; → polarimetric; → high; → contrast; → exoplanet.

The symmetry plane of the → Local Supercluster, where density of galaxies in our environment is the largest. The plane passes through the → Virgo cluster of galaxies, about which many of the brightest galaxies in the sky are concentrated. The supergalactic plane was recognized by Gérard de Vaucouleurs (1918-1995) in 1953 from the  → Shapley-Ames catalogue.

→ supergalactic; → plane.

A planet whose orbit lies outside that of the Earth. The superior planets are Mars, Jupiter, Saturn, Uranus, and Neptune. → planet.

→ superior; → planet.

The four innermost planets in the solar system, Mercury, Venus, Earth and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth's. The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none. These planets are approximately the same size, with the Earth the largest. They are considerably denser than the Jovian planets, ranging from a specific gravity of 4 for Mars to 5.5 for the Earth.

→ terrestrial; → planet.