A fusor, according to a proposal to the IAU by Gibor Basri, Professor of Astronomy at the University of California, Berkeley to help clarify the nomenclature of celestial bodies, is "an object that achieves core fusion during its lifetime". This definition included any form of nuclear fusion, so the lowest possible mass of a fusor was set at roughly 13 times that of Jupiter, at which point deuterium fusion becomes possible. This is significantly smaller than the point at which sustained hydrogen fusion becomes possible, around 60 times the mass of Jupiter. Objects are considered "stellar" when they are about 75 times the mass of Jupiter, when gravitational contraction, i.e. contraction of the object due to gravity, is halted by heat generated by the nuclear reaction in their interiors. Fusors would include active stars, dead stars, and brown dwarfs.
The introduction of the term "fusor" would allow for a simple definition:
where round is understood as "whose surface is very nearly on the gravitational equipotential", orbits means "whose primary orbit is now, or was in the past around", and capable implies fusion is possible sometime during the existence of the object by itself.
In ancient times, only the Sun and Moon, a few stars, and the most easily visible planets had names. Over the last few hundred years, the number of identified astronomical objects has risen from hundreds to over a billion, and more are discovered every year. Astronomers need to be able to assign systematic designations to unambiguously identify all of these objects, and at the same time give names to the most interesting objects and, where relevant, features of those objects.
The International Astronomical Union (IAU) is the officially recognized authority in astronomy for assigning designations to celestial bodies such as stars, planets, and minor planets, including any surface features on them. In response to the need for unambiguous names for astronomical objects, it has created a number of systematic naming systems for objects of various sorts.Fusor (disambiguation)
Fusor or Fuzor may refer to:
FusorFusor or the Farnsworth–Hirsch Fusor, an apparatus designed by Philo T. Farnsworth to create nuclear fusion
Fusor (astronomy), an object that achieves core fusion during its lifetimeFuzorFuzors, a Transformers sub-line (toys, comics and cartoons)
Zoids Fuzors, an animeIAU definition of planet
The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body which:
is in orbit around the Sun,
has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
has "cleared the neighborhood" around its orbit.Among other things, this definition caused Pluto to no longer be a planet, a change from how it had been widely considered until that point.
A non-satellite body fulfilling only the first two of these criteria (such as Pluto) is classified as a "dwarf planet". According to the IAU, "planets and dwarf planets are two distinct classes of objects". A non-satellite body fulfilling only the first criterion is termed a "small Solar System body" (SSSB). An alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The definition was a controversial one, and has drawn both support and criticism from different astronomers, but has remained in use.
According to this definition, there are eight known planets in the Solar System. The definition distinguishes planets from smaller bodies and is not applicable outside the Solar System. To date, there is no accepted definition of extrasolar planets, or exoplanets. In 2007, an IAU working group issued a position statement that proposes to distinguish exoplanets from brown dwarfs on the basis of mass, but there has been no IAU-wide resolution or vote associated with this position statement. A separate proposal to extend the IAU definition to exoplanets has not been formally reviewed by the IAU.Mesoplanet
Mesoplanets are planetary bodies with sizes smaller than Mercury but larger than Ceres. The term was coined by Isaac Asimov. Assuming size is defined in relation to equatorial radius, mesoplanets should be approximately 500 km to 2,500 km in radius.
The term was coined in Asimov's essay "What's in a Name?", which first appeared in The Los Angeles Times in the late 1980s and was reprinted in his 1990 book Frontiers; the term was later revisited in his essay, "The Incredible Shrinking Planet" which appeared first in The Magazine of Fantasy and Science Fiction and then in the anthology The Relativity of Wrong (1988).Asimov noted that the Solar System has a large number of planetary bodies (as opposed to the Sun and natural satellites) and stated that lines dividing "major planets" from minor planets were necessarily arbitrary. Asimov then pointed out that there was a large gap in size between Mercury, the smallest planetary body that was considered to be undoubtedly a major planet, and Ceres, the largest planetary body that was considered to be undoubtedly a minor planet. Only one planetary body known at the time, Pluto, fell within the gap. Rather than arbitrarily decide whether Pluto belonged with the major planets or the minor planets, Asimov suggested that any planetary body that fell within the size gap between Mercury and Ceres be called a mesoplanet, because mesos means "middle" in Greek.Protoplanet
A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationally perturb each other's orbits and collide, gradually coalescing into the dominant planets.
In the case of the Solar System, it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos were similar to Ceres and Pluto with masses of about 1022 to 1023 kg and were a few thousand kilometers in diameter. Over the course of hundreds of millions of years, they collided with one another. The exact sequence whereby planetary embryos collided to assemble the planets is not known, but it is thought that initial collisions would have replaced the first "generation" of embryos with a second generation consisting of fewer but larger embryos. These in their turn would have collided to create a third generation of fewer but even larger embryos. Eventually, only a handful of embryos were left, which collided to complete the assembly of the planets proper.Early protoplanets had more radioactive elements, the quantity of which has been reduced over time due to radioactive decay. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavier elements sank to the center, whereas lighter elements rose to the surface. Such a process is known as planetary differentiation. The composition of some meteorites show that differentiation took place in some asteroids.
According to the giant impact hypothesis the Moon formed from a colossal impact of a hypothetical protoplanet called Theia with Earth, early in the Solar System's history.
In the inner Solar System, the three protoplanets to survive more-or-less intact are the asteroids Ceres, Pallas, and Vesta. Psyche is likely the survivor of a violent hit-and-run with another object that stripped off the outer, rocky layers of a protoplanet. The asteroid Metis may also have a similar origin history to Psyche. The asteroid Lutetia also has characteristics that resemble a protoplanet. Kuiper-belt dwarf planets have also been referred to as protoplanets. Because iron meteorites have been found on Earth, it is deemed likely that there once were other metal-cored protoplanets in the asteroid belt that since have been disrupted and that are the source of these meteorites.
In February 2013 astronomers made the first direct observation of a protoplanet forming in a disk of gas and dust around a distant star.