Venus transit and lunar mirror could help astronomers find worlds around other stars

Jan 07, 2013
The Transit of Venus of 6 June 2012, as seen from Wiltshire in the UK. Credit: Grant Privett

(Phys.org)—On 6 June 2012 Venus passed directly between the Earth and the Sun, in a so-called transit where the planet appears as a silhouette against the solar disk, something that will not happen again until 5 December 2117. A team of Italian astronomers led by Paolo Molaro of the Instituto Nazionale di Astrofisica at the University of Trieste used the opportunity to perform an unusual and challenging experiment, looking at the sunlight reflected off the Moon ('moonlight') to see how it changed during the transit. This technique could help scientists to find planets in orbit around other stars. The team publish their results in the journal Monthly Notices of the Royal Astronomical Society: Letters.

When Venus passes in front of the Sun it hides a part of our star's rotating surface. Because of rotation, the spectrum of the Sun (created splitting the different colours of light using a ) is slightly different on each side. On one side, the is rotating towards the observer and so its light will be 'blueshifted', meaning the lines seen in a spectrum move towards shorter wavelengths. On the other, the surface is rotating away from the observer, so its light is 'redshifted', meaning that the lines move towards longer wavelengths.

By looking at the reflected light from the , this is averaged out as a broadening of the various lines. When Venus moves in front of the Sun from east to west, it first blocks out the surface moving towards us and then the surface moving away from us. This causes a distortion in the spectral lines known as the "Rossiter-McLaughlin effect".

The astronomers realised that the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph installed on a 3.6m telescope at La Silla in Chile, part of the (ESO), would be sensitive enough to detect the effect and that the Moon would be in the right place too. The Moon was slightly ahead of the Earth in its orbit, so 'saw' the transit a couple of hours later than terrestrial observers. This also meant that 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.

Distortions in the resulting from the Rossiter-McLaughlin effect are extremely small and are equivalent to a 3km / hour shift in the observed motion of the Sun. It has been seen before, in binary systems where the two stars eclipse each other. But it becomes more and more difficult to observe when the celestial body is a planet and rather than being the size of Jupiter is similar in size to the Earth, as Venus is. Scientists should nonetheless be able to measure the extent of this weak effect on the light from other planetary systems using telescopes such as the European Extremely Large Telescope (E-ELT) now under development. With the technique it should be possible to characterise important features of these systems and thus improve our understanding of the formation of planets in general.

Team member Lorenzo Monaco from ESO describes how important HARPS was in their work. "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 kilometres, the space that separates us from the Sun. Nowadays there are very few instruments capable of recording such tiny changes, especially if you only have a few hours to measure them".

"There is close agreement between our work and the theoretical models", says Mauro Barbieri, from University of Padua, who is also a member of the team. "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".

"This measurement", says Paolo Molaro "shows the sensational results that spectrographs on telescopes like E-ELT will be able to deliver. We will open a new horizon in the study of the other Earth-like planets that are almost certainly waiting to be found around other stars in our galaxy".

Explore further: Astronomers find 'cousin' planets around twin stars

More information: The new work appears in, "Detection of the Rossiter–McLaughlin effect in the 2012 June 6 Venus transit", P. Molaro, L. Monaco, M. Barbieri, S. Zaggia, Monthly Notices of the Royal Astronomical Society: Letters, published online at mnrasl.oxfordjournals.org/cont… l.sls027.short?rss=1
This paper is currently free to access.

Related Stories

Hubble to use moon as mirror to see Venus transit

May 04, 2012

This mottled landscape showing the impact crater Tycho is among the most violent-looking places on our moon. Astronomers didn't aim NASA's Hubble Space Telescope to study Tycho, however. The image was taken ...

Venus to appear in once-in-a-lifetime event

May 01, 2012

On 5 and 6 June this year, millions of people around the world will be able to see Venus pass across the face of the Sun in what will be a once-in-a-lifetime experience.

ESA missions gear up for transit of Venus

Jun 04, 2012

(Phys.org) -- ESA’s Venus Express and Proba-2 space missions, along with the international SOHO, Hinode, and Hubble spacecraft, are preparing to monitor Venus and the Sun during the transit of Earth’s ...

ND expert: The science behind the transit of Venus

May 30, 2012

University of Notre Dame professor of physics Peter Garnavich has research interests that cover a wide range of topics in observational astrophysics. In preparation for the Tuesday (June 5) Transit of Venus, he offers an ...

Recommended for you

The origins of local planetary orbits

15 hours ago

A plutino is an asteroid-sized body that orbits the Sun in a 2:3 resonance with Neptune. They are named after Pluto, which also orbits the Sun twice for every three orbits of Neptune. It is thought that Pluto ...

Wild ducks take flight in open cluster

16 hours ago

The Wide Field Imager on the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory in Chile has taken this beautiful image, dappled with blue stars, of one of the most star-rich open clusters currently ...

Image: The Pillars of Creation

17 hours ago

The Pillars of Creation (seen above) is an image of a portion of the Eagle nebula (M16) taken by Hubble Space Telescope in 1995. It soon became one of the most iconic space images of all time. The Eagle nebula ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

that_guy
1 / 5 (2) Jan 07, 2013
Seemed much more interesting before I read the article. I respect the science and ingenuity that went into this experiment, but essentially we are talking about pairing transit planetfinding - Which can only detect a very small percentage of planets due to geometric and timing limitations - with a less direct and less effective measurement/detection.

This can only provide a very mild and small amount of possible additional information.