to have achieved hydrostatic equilibrium (the so-called planetary-mass moons); they would be considered planets if only the physical parameters are considered. Both Jupiter's moon Ganymede and Saturn's moon Titan are larger than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as Io and Triton demonstrate obvious and ongoing geological activity, and Ganymede has a magnetic field. Just as stars in orbit around other stars are still referred to as stars, some astronomers argue that objects in orbit around planets that share all their characteristics could also be called planets. Indeed, Mike Brown makes just such a claim in his dissection of the issue, saying: It is hard to make a consistent argument that a 400 km iceball should count as a planet because it might have interesting geology, while a 5000 km satellite with a massive atmosphere, methane lakes, and dramatic storms [Titan] shouldn't be put into the same category, whatever you call it. However, he goes on to say that, "For most people, considering round satellites (including our Moon) 'planets' violates the idea of what a planet is." Alan Stern has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term satellite planet for planetary-mass moons. Extrasolar planets and brown dwarfs The discovery since 1992 of extrasolar planets, or planet-sized objects around other stars ( such planets in planetary systems including multiple planetary systems as of ), has widened the debate on the nature of planethood in unexpected ways. Many of these planets are of considerable size, approaching the mass of small stars, while many newly discovered brown dwarfs are, conversely, small enough to be considered planets. The material difference between a low-mass star and a large gas giant is not clear-cut; apart from size and relative temperature, there is little to separate a gas giant like Jupiter from its host star. Both have similar overall compositions: hydrogen and helium, with trace levels of heavier elements in their atmospheres. The generally accepted difference is one of formation; stars are said to have formed from the "top down", out of the gases in a nebula as they underwent gravitational collapse, and thus would be composed almost entirely of hydrogen and helium, while planets are said to have formed from the "bottom up", from the accretion of dust and