Why Are Planets Sphere?

Stars are perfect examples of natural spheres. the mass of a star is mind-boggling, large and creates  and equally mind-boggling large amount of gravity. What shape does Mother Nature give to so much mass to minimize its enormous value? A sphere.

(Stars like our Sun are huge, dynamic, energy-producing concentrations of Hydrogen and Helium,  compacted by their enormous gravity into spheres.)

The reason planets appear so spherical is because gravity compresses the planet into a shape that most evenly distributes the gravitational force among the planet's mass.
Whether it is shaping water droplets, stars, soap bubbles or planets, nature seeks to minimize the surface area needed to contain a given volume, and the shape that keeps volume at the absolute minimum a sphere.
Any object in weightless space larger than a couple of hundred miles in diameter has enough mass for its gravity to overcome large-scale irregularities and force it into a spherical shape. The gravitational compression also generates significant amounts of heat at the center of the planet. This heat melts, or at least soften, any solid materials within the planet, facilitating the planet's collapse into a sphere.
Objects in space smaller than about 100 miles in diameter, such as most asteroids, comet nuclei, and small moons, lack the mass to create a gravtational field strength strong enough to compress themselves into spheres. These little spheres are often taken on what I call the 'sick potato' look.

(The 12.5 miles long, 7.5 mile wide asteroid Gaspra, imaged October 1991 from a distance of 1600 miles  by the Galelio spacecraft en route to Jupiter)

A really large asteriod, such as Ceres (diameter = 600 miles), has enough mass for its gravity to compress it into a sphere.

(The 600 mile-wide asteroid Ceres as seen by the Hubble Space Telescope)

However, 'perfect' spheres are hard to find in space.

Pretty much everything in space rotates, and a rotating non-rigid sphere causes it to 'bulge' at its equator from the centrifugal forces acting on it.

This spinning distorted large planets into a slightly squashed shape known as an 'oblate spheroid'. This means that a planet's diameter measured through its poles is smaller than the diameter measured through its equator.

Whereas the difference between the polar diameter and the equatorial diameter of Earth is a barely noticeable 0.3%, the oblateness of Saturn, a large, gaseous and rapidly spinning planet, is greater than 10%. You can easily see Saturn's polar flattening through a telescope.

(Saturn's polar diameter is 33,700 miles, but its equatorial diameter is 37,360 miles) 

Is The Earth Getting Smaller?

There are factors that are causing Earth to both gain and lose mass over time, according to Dr Chris Smith, a medical microbiologist and broadcaster who tries to improve the public understanding of science.

Using some back-of-the-envelope-style calculations, Dr Smith, with help from physicist and Cambridge University colleague Dave Ansell, drew up a balance sheet of what’s coming in, and what’s going out. All figures are estimated.

By far the biggest contributor to the world’s mass is the 40,000 tonnes of dust that is falling from space to Earth, says Dr Smith.

“[The dust] is basically the vestiges of the solar system that spawned us, either asteroids that broke up or things that never formed into a planet, and it’s drifting around.

“The Earth is acting like a giant vacuum cleaner powered by gravity in space, pulling in particles of dust,” says Dr Smith.

Another much less significant reason the planet is gaining mass is because of global warming.

“Nasa has calculated that the Earth is gaining about 160 tonnes a year because the temperature of the Earth is going up. If we are adding energy to the system, the mass must go up,” says Dr Smith.

This means that in total between 40,000 and 41,000 tonnes is being added to the mass of the planet each year.

Population growth and new buildings are not a factor, he says, because both of these are actually made up of existing matter on the planet.

But overall, Dr Smith has calculated that the Earth - including the sea and the atmosphere - is losing mass. He points to a handful of reasons.

For instance, the Earth’s core is like a giant nuclear reactor that is gradually losing energy over time, and that loss in energy translates into a loss of mass.

But this is a tiny amount - he estimates no more than 16 tonnes a year.

And what about launching rockets and satellites into space, like Phobos-Grunt? Dr Smith discounts this as most of it will fall back down to Earth again.

But there is something else that is making the planet lose mass. Gases such as hydrogen are so light, they are escaping from the atmosphere.

“Physicists have shown that the Earth is losing about three kilograms of hydrogen gas every second. It’s about 95,000 tonnes of hydrogen that the planet is losing every year.

“The other very light gas this is happening to is helium and there is much less of that around, so it’s about 1,600 tonnes a year of helium that we lose.”

So taking into account the gains and the losses, Dr Smith reckons the Earth is getting about 50,000 tonnes lighter a year, which is just less than half the gross weight of the Costa Concordia, the Italian cruise liner, that ran aground recently.

Summary:

• It’s getting lighter, by about 50,000 tonnes in mass each year, but not due to space dust
• Some factors include:
• Gains: Mostly dust (like an asteroid, above) falling from space, plus increased energy from increases in the planet’s temperature
• Losses: Mostly hydrogen, plus some helium and a tiny amount of lost energy