the feel of density in the EARTH sense sector
a article on Gliese 581 c
In spectroscopy, spectral flux density is the quantity that describes the rate at which energy is transferred by electromagnetic radiation through a real or virtual surface, per unit surface area and per unit wavelength. It is a radiometric measure, as distinct from measures that characterize light in terms of the electromagnetic field or photons. In SI units it is measured in W m-3, though it is more practical to measure it in W m-2 nm-1 (1 W m-2 nm-1 = 1 GW m-3 = 1 W mm-3) or W m-2 μm-1 (1 W m-2 μm-1 = 1 MW m-3).
The terms used to describe spectral flux density vary between fields, sometimes including adjectives such as “electromagnetic” or “radiative”, and sometimes dropping the word “density”. Applications include:
Characterizing remote telescopically unresolved sources such as stars, observed from a specified observation point such as an observatory on earth.
Characterizing the electromagnetic radiative field at a point, measured there with an instrument that collects radiation from a whole sphere or hemisphere of remote sources.
Gliese 581 c (pronounced /ˈɡliːzə/) or Gl 581 c is a planet orbiting the red dwarf star Gliese 581. It is the second planet discovered in the system and the third in order from the star. With a mass at least 5.6 times that of the Earth, it is classified as a super-Earth, a category which incorporates planets exceeding the mass of Earth but smaller than 10 Earth masses. Assuming that the planet’s mass is close to the lower limit determined by radial velocity measurements (the true mass is unknown), it was the smallest known extrasolar planet around a main sequence star, but on April 21, 2009, another planet orbiting Gliese 581, Gliese 581 e, was announced with an approximate mass of 1.9 earth masses, which is now the smallest known extrasolar planet around a main sequence star.
Gliese 581 c initially generated interest because it was originally reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface, and by extension, potentially capable of supporting extremophile forms of Earth-like life. However, further research on the potential effects of the planetary atmosphere casts doubt upon the habitability of Gliese 581 c and indicates that the fourth planet in the system, Gliese 581 g, is a better candidate for habitability. In astronomical terms, the Gliese 581 system is relatively close to Earth, at 20.3 light years (192 trillion km or 119 trillion miles) in the direction of the constellation of Libra. This distance, along with the declination and right ascension coordinates, give its exact location in our galaxy. It is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars; it is the 87th closest known star system to the Sun.