Radar observations in 1965 demonstrated instead that Mercury has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun, which results in the same positioning at those observation points. Modeling has demonstrated that Mercury was captured into the 3:2 spin–orbit state very early in its history, within 20 (and more likely even 10) million years after its formation. The 583.92-day interval between successive close approaches of Venus to Earth is equal to 5.001444 Venusian solar days, making approximately the same face visible from Earth at each close approach. Whether this relationship arose by chance or is the result of some kind of tidal locking with Earth is unknown. The exoplanet Proxima Centauri b, discovered in 2016 that orbits around Proxima Centauri, is tidally locked, expressing either synchronized rotation or a 3:2 spin–orbit resonance like that of Mercury. One form of hypothetical tidally locked exoplanets are eyeball planets, which in turn are divided into "hot" and "cold" eyeball planets. Close binary stars throughout the universe are expected to be tidally locked with each other, and extrasolar planets that have been found to orbit their primaries extremely closely are also thought to be tidally locked to them. An unusual example, confirmed by MOST, may be Tau Boötis, a star that is probably tidally locked by its planet Tau Boötis b. If so, the tidal locking is almost certainly mutual. However, since stars are gaseous bodies that can rotate with a different rate at different latitudes, the tidal lock is with Tau Boötis's magnetic field. An estimate of the time for a body to become tidally locked can be obtained using the following formula:  where  formula_12 and formula_13 are generally very poorly known except for the Moon, which has formula_14. For a really rough estimate it is common to take formula_15 (perhaps conservatively, giving overestimated locking times), and where  Even knowing the size and density of the satellite leaves many parameters that must be estimated (especially "ω", "Q", and "μ"), so that any calculated locking times obtained are expected to be inaccurate, even to factors of ten. Further, during the tidal locking phase the semi-major axis formula_20 may have been significantly different from that observed nowadays due to subsequent tidal acceleration, and the locking time is extremely sensitive to this value. Because the uncertainty is so high, the above formulas can be simplified to give a somewhat less cumbersome one.