“People
scratch their heads about microlensing. This will show the power of the
technique, get people excited and involved,” said NASA’s astronomer Calen
Henderson in this week’s Science. But what is microlensing exactly? And how can
Kepler use it?
 |
The ring of light around the star comes from another
source behind the star in the middle. The ‘gravity pit’ of the star has bent the light. |
Microlensing
is, like many cool things in physics, a phenomena caused by Einstein’s theory
of general relativity. This theory states that mass curves space time, like a
heavy rock curves a rubber sheet. This is the reason why, for example, planets
orbit stars; they are ‘trapped’ in the ‘pit’ the star made in space-time, like
a marble which rolls over the rubber sheet will orbit the rock too, if it has
the right speed. But a funny thing is, light is also affected by this ‘pit’ in
space-time. Because light is incredibly fast, it doesn’t get trapped in the
pit, but it changes direction if it passes a heavy object, like a marble which
rolls really fast past the rock. Since light also changes direction if it
travels through a lens, this is called gravitational lensing. This effect is of
course bigger when the object which acts as a lens, the rock, is heavier, if
it’s for example a whole galaxy, because the light gets bent more then. But it
is also detectable on a much smaller scale, like with a star acting as lens, or
even a planet. This is called microlensing, and that’s exactly what Kepler is
going to use.
But what’s Kepler currently doing?
Kepler is a
space probe that orbits the sun and searches for planets that orbit other
stars. It has already discovered many planets using a method called the transit
method. This method uses the fact that a planets throws a little shadow on its
star as it passes in front of it. This causes a slight decrease in the
brightness of the star. Kepler can detect this and then deduce the size, mass
and other things of the planet. Kepler has already discovered many planets
using the transit method, but there’s a problem. With the transit method,
Kepler can’t detect planets which are really far away from their star or
planets that don’t orbit a star at all. This is where microlensing comes into
play.
The effect
of microlensing is so small that a ring, like in the picture, doesn’t appear. A
star or planet simply can’t bent the light that much. Instead, the star behind
the ‘lens’ appears brighter. If a star that has an orbiting planet moves in
front of another star that is really far away, the far-away star will become
brighter, because the light gets bended by the star’s gravity. But if the
planet also moves in front of the far-away star, the planet also bends the
light a little, making the far-away star even brighter.
This method
is harder to use than the transit method, because it also requires telescopes
on earth working together with Kepler. A planet passing in front of its star
also happens way more often than a star with a planet passing in front of a
far-away star. Kepler will probably discover way less planets with microlensing
than it did with the transit method. But microlensing can discover planets that
the transit method can’t. Planets that don’t orbit a star can be found with
microlensing, but not with the transit method, for example. The reason for this
is very simple. You can’t throw a shadow on your star if you don’t have one. Kepler
also has trouble detecting planets that orbit their stars very slowly, since
the planets won’t pass in front of their star very often and a pattern would be
difficult to find. Mircolensing doesn’t have these kinds of problems so it is a
great method for discovering planets we couldn’t detect before. This means we
can find out even more about the universe!
Sources:
https://en.wikipedia.org/wiki/Gravitational_lens#/media/File:A_Horseshoe_Einstein_Ring_from_Hubble.JPG
No comments:
Post a Comment
Great that you want to comment! Please write something relevant and non-offending.