Using $E=mc^2$ that works out to about $1.4 \times 10^{22}$ Joules/year or $4.6 \times 10^{14}$ Watts. That's about 3.6 Watts/meter^2 or about 0.3% of the total power coming from the Sun, so it does seem to be back-of-the-envelope-in-the ballpark correct!

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.