♇ Pluto

Pluto is no longer technically a planet — it was ‘downgraded’ to dwarf planet status in 2006. Until 2015 all we had were highly pixellated images of it. But in 2015 the New Horizons space probe sent back stunning images, and the whole world fell in love with Pluto.

But the decision to strip it of planet status was probably the right one. There are many ways in which it is different from the eight planets we now accept.

For example, its orbit is more eccentric than the eight planets (0.25, compared with 0.21 for Mercury, the most eccentric planet, with the others all being less than 0.1). Indeed, its orbit is so eccentric that it is sometimes closer to the Sun than Neptune, despite Pluto being on average 39 AU away from the Sun, compared with Neptune’s average distance of 30 AU.

Its orbital inclination to the ecliptic is 17° (compared with 7° for Mercury, and under 3.5° for the remaining planets).

The centre of mass of Pluto and its largest moon, Charon, lies outside Pluto itself. In other words Pluto and Charon both orbit a point that lies between them. For this reason they are sometimes considered a twin-planet system. (The centre of mass of the Earth-Moon system is well inside the Earth, so that the Moon can more properly be considered to be orbiting the Earth.)

STATISTICS

ORBITAL RADIUS

5.9 billion km from the Sun (39 times the distance of the Earth from the Sun)

ORBITAL PERIOD

248 Earth years

DIAMETER

2,400 km (0.19 times the diameter of the Earth)

ROTATIONAL PERIOD

6.4 Earth days

MASS

0.2% of the mass of the Earth, 18% of the mass of the Moon

SURFACE GRAVITY

about one-fifteenth of the gravitational force on Earth

ALBEDO

0.3

MOONS

Pluto is known to have (at least) five moons.

The largest moon is Charon, which has a diameter of 1,200 km and orbits at a distance of 20,000 km. Its mass is around one-eighth the mass of Pluto (whereas the mass of the Moon is about one-hundredth the mass of the Earth).

The name comes from the name of the ferryman of the dead from Greek mythology. (Pluto being the Roman god of the underworld.) For this reason the correct pronunciation should be care-on. However, James Christy, who discovered Charon, proposed the name as a variant of his wife’s name, Charlene. In which case the correct pronunciation is share-on.