December 14, 2012
Lunar Light During Venus Transit Could Help Astronomers Observe Earth-Like Planets
Lee Rannals for redOrbit.com - Your Universe Online
Astronomers reported in the journal Monthly Notices of the Royal Astronomical Society that looking at the sunlight reflected off the Moon during the Venus transit this year helped scientists find planets in orbit around other stars.As Venus passed in front of the Sun on June 6, 2012, it hid part of our sun's rotating surface. Because of rotation, the spectrum of the Sun is slightly different on each side.
On one side of the sun, the solar surface is rotating towards the observer and its light is "blue shifted," which means the lines seen in a spectrum move towards shorter wavelengths.
The other side of the sun has a surface rotating away from the observer, so its light is "redshifted," meaning that the lines move towards longer wavelengths.
When looking at the reflected light from the lunar surface, this is averaged out as a broadening of the various lines. When Venus transited in front of the Sun from east to west, it first blocked out the surface moving towards us, and then the surface moving away from us, which caused the "Rossiter-McLaughlin effect."
Italian astronomers used the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph installed on a telescope at La Silla in Chile to detect the effect.
Because the Moon was slightly ahead of the Earth in its orbit, the "transit" took place there a couple of hours later than terrestrial observers. This meant the Moon was in the night time sky in Chile, making it possible for the La Silla telescope to operate safely and observe the change in the solar spectrum.
Distortion in the spectral lines resulting from the Rossiter-McLaughlin effect are extremely small and equivalent to a 1.86-mile per hour shift in the observed motion of the Sun.
In binary systems, where the two stars eclipse each other, this effect becomes more and more difficult to observe when the celestial body is a planet.
Scientists believe they will be able to measure the extent of this weak effect on the light from other planetary systems using telescopes like the European Extremely Large Telescope.
Using the technique, astronomers will be allowed to characterize important features of these systems, helping to improve scientists' understanding of the formation of planets.
"The measured magnitude of the Rossiter-McLaughlin effect is comparable to being able to track the speed of a person walking at a slow pace at a distance of 150 million kilometers, the space that separates us from the Sun," said team member Lorenzo Monaco from the ESO. "Nowadays there are very few instruments capable of recording such tiny changes, especially if you only have a few hours to measure them".
Mauro Barbieri, from University of Padua, who is also a member of the team, said that there is close agreement between the astronomers' work and theoretical models.
"Among other things, this change in velocity is comparable with that due to the natural expansion and contraction of our star. Our observations however have allowed us to clearly see the Rossiter-McLaughlin effect during the transit," Barbieri said.
Team member Paolo Molaro said that this measurement will open a new horizon in the study of other Earth-like planets that almost certainly want to be found around other stars in the Milky Way.