In 1911, Albert Einstein predicted that mass can act to bend the path of
light through space, which he then went on to explain, in 1915, by
explaining that matter acts to bend space itself. The more matter, the
more mass, the more space is bent. Imagine a giant rubber sheet, pulled
tight over a hole (effectively making a giant drum!). With nothing on the top, the
sheet would be flat, but if you place a heavy object on it (such as an
apple), that object will cause a dent in the sheet, pulling it lower in
the area around the object. The more massive the object, the greater
the distortion, and the deeper the hole. Now, picture yourself attempting
to roll marbles from one side of the sheet to the other, past the dip.
As the marble enter the dip, they are deflected by an amount depending
on the steepness of the dent. The deeper the hole, and the closer to
the centre the marbles come, the greater the deflection!
Strange as it sounds that mass can bend space, and that this can curve beams of
light, it does actually happen! Just eight years after Einstein made
his initial prediction, British scientists travelled to observe a total
Solar eclipse, with the aim of measuring the positions of stars near the Sun in the
sky with great accuracy. Despite bad weather, and problems with their
equipment, the scientists managed to obtain their measurements, which
showed that the stars were in slightly different positions to those you
would normally expect, if the Sun wasn't nearby. Their light had indeed
been bent, by just the amount that Einstein predicted!
This quirky behaviour of light is used today by astronomers to look for
planets far beyond our Solar system. The technique known as
"Gravitational microlensing" exploits the transient
brightening of a distant star caused by the gravitational bending of
its light when another star drifts in front of it. The motions of stars
within our Galaxy are such that these chance alignments last for about
a month. If the foreground star has planets orbiting it, these
further modify the curvature of space slightly, which can lead to a
small “blip” (lasting between hours and days) in the observed light
curve (a light curve is simply a plot showing how the brightness of the
star varies with time). If you want to know more about microlensing,
check out this link.
In contrast to the other methods (winks and wobbles), microlensing observations can
provide a census of planets orbiting distant stars within the Milky
Way, rather than being restricted to the Solar neighbourhood. It is
also sensitive to planets that are far from their host star: these
planets take many earth-years to complete a single orbit, and
consequently are not yet being detected by other means. Microlensing
allows astronomers to find a new world in just one night, but we will
never encounter a sign of it again.
The above video tells the story of the discovery of the icy planet OGLE-2005-BLG-390Lb by microlensing,
as reported on the BBC news.
To date, scientists have detected six distant planets by looking for
microlensing events. However, since the longer we look, the more such
events there will be, this number should continue to climb, potentially
yielding thousands, or even millions of planets!