Forget simply detecting a slight “dip” in brightness as an exoplanet transits in front of its star, soon we’ll be able to image the event. What’s more, by doing this we’ll see that exoplanetary transits look exactly like the historic Venus transit that wowed the world on Tuesday.
This is according to astronomer Gerard van Belle, of Lowell Observatory near Flagstaff, Ariz., who hopes to use an interferometer to carry out the mind-blowing goal of capturing the silhouettes of exoplanets drifting in front of distant stars. But that’s not all, this whole effort may help us track down the first bona fide Earth-like alien world.
“We’re not just trying to take pictures of stars and see them as disks — which is something we can do — but the feather in our cap is to image one of these transit events and it would look just like the Venus event,” van Belle told Discovery News. “This is something that’s just coming to within our grasp.”
NASA’s Kepler space telescope looks for the tell-tail signs of exoplanetary transits by detecting the slight dimming of a star. This dimming is caused by an exoplanet passing between the star and Kepler — the amount of dimming corresponds to the exoplanet’s physical size and the length of dimming corresponds to its orbital characteristics. Although Kepler is sensitive to Earth-sized worlds, it isn’t designed to directly image an exoplanet’s silhouette (as shown in the artist’s impression, top-right).
Interferometers, on the other hand, are a number of telescopes that are linked and their combined observational power can mimic a larger telescope with the equivalent diameter the distance that separates them. The Navy Optical Interferometer at Lowell Observatory or the CHARA (Center for High Angular Resolution Astronomy) array atop Mt. Wilson, Calif., are two interferometer systems that could achieve direct imaging of transits, says van Belle.
Also, in the aim of using the Venus transit to educate the world on what Kepler ‘sees’ when it detects exoplanet transits, the Venus transit turned out to be the ideal exoplanetary “analog.” He added: “I think it’s really timely that we should have this Venus event, so that if we get to the point of being able to accomplish (directly imaging an exoplanetary transit) in the next year, three years or so, people can say, ‘oh yeah, that looks just like the Venus event except it’s around another star!’”
But the connection between the Venus transit and exoplanet transits go far beyond just “looking the same”; valuable science was done during this week’s Venus transit — the last such event until the year 2117, in 105 years time.
As Venus drifted in front of the sun’s disk, it blocked a tiny amount of sunlight. However, it also refracted a smaller amount of sunlight through its atmosphere.
Certain molecules in Venus’ upper atmosphere would have absorbed certain wavelengths of light, so by using very sensitive spectrometers — instruments that split light into its component wavelengths — absorption lines that correspond to the presence of certain components of the Venusian atmosphere should have been detected.
“For Lowell Observatory, we have a spectrograph on one of our telescopes — the 42 inch Hall Telescope — that is designed to take light from a fiber and you can feed that into your instrument to do science. It splits the light up into its different colors,” van Belle continued. “You can look for calcium lines and other sorts of elements in the spectra.
“With that fiber on the delivering end, you can either attach it to the back end of the telescope, where at night time it’s used to look at the stars. But in the daytime you just run the fiber outside and point the end at the sun — just the tip of the fiber has enough collecting area. We are the proud owners of the smallest optical telescope!”
Basically, the same spectrometer can be used to analyze the light from a very dim and distant star (with the Hall Telescope attached) or the light from our nearest star (without the telescope attached). Although the like-for-like comparison sounds like a great way to do science, van Belle cautioned that the spectra from refracted light through Venus’ atmosphere may be too weak for the Lowell spectrometer to detect (although the data collected still needs extensive analysis).
But why go through the painstaking analysis of trying to analyze light refracted by Venus’ atmosphere in the first place? We know the composition of the Venus atmosphere very well, why would we need to analyze it from from Earth?
We’ve landed robots on the hellish Venusian surface and the European Space Agency currently has a spacecraft in orbit around the planet — Venus Express. We therefore have in-situ and orbital measurements of the planet’s atmospheric composition. It is for this reason that observations of the Venus transit are so important.
If the analysis of the Venus transit’s spectrum detects components of Venus’ atmosphere, and they are in agreement with measurements made by Venus Express (for example), then we can learn a method of detecting the spectroscopic signature of exoplanetary atmospheres when we analyze starlight during exoplanetary transits. Venus, once again, is acting like a close-up exoplanet that we can study in the aim of honing our skills to detect the atmospheric composition of distant alien worlds.
“This is really the direction the whole game is going in with planets around other stars — not just finding them; just a short while ago this was a big fat deal! Nowadays we’re careening headlong into that being passé,” said van Belle.
“Now the new name of the game is actually full-blown characterization of these planets; being able to establish how big they are, establish what their masses are, figure out what the atmospheres are made out of — do they have a lot of water, carbon dioxide, or other more interesting things?
“All of this is getting to a far grander game of getting to very small planets that are about the size of Earth and we can detect signatures that we think are consistent with life.”
So that is how the Venus transit may contribute to the ultimate goal of finding a true “Earth-like” planet orbiting another star. After all, an exoplanet is not truly “Earth-like” until we can establish whether or not its atmosphere can support life as we know it.
Image: Left: A photograph of the Venus transit on June 5, 2012 (Ian O’Neill). Right: An artist’s impression of an exoplanet transit (ESO).