Editorial: “Transit of Venus and the changing nature of discovery“
VENUS, named after the goddess of love, is a tease. She swans around between the sun and Earth in an orbit that brings her closer to our planet than any other, and shines more brightly than anything in the night sky aside from the moon. Yet she remains frustratingly mysterious, in part because she is cloaked in thick clouds that for centuries left her surface to the imagination. As recently as the 1950s, some astronomers argued that the planet next door – which is – was a balmy water world that could host life.
Spacecraft sent to Venus put paid to that fantasy. They revealed a carbon dioxide-choked planet where the atmospheric pressure on the surface is 90 times that of Earth and the 460°C average temperature is hot enough to melt lead. Rather than the wholesome beauty of a Botticelli painting, Venus suddenly seemed more like a femme fatale.
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“That was the start of the decline in interest in Venus,” says , who works at the European Space Agency in Noordwijk, the Netherlands. “Because of the extremely high temperatures, the chance of finding life on Venus was more remote, and people’s interest turned more towards Mars.” In the past two decades, the world’s space agencies have launched , yet a mere two to Venus.
But now the tables have turned. For nearly 7 hours this week, thanks to a rare celestial alignment, all eyes will be on our nearest neighbour. On 5 or 6 June, depending on your location, Venus will glide in front of the sun, appearing as a small dark spot that crawls across the star’s face. Such transits are exceedingly rare because Venus and Earth do not orbit in exactly the same plane. Only six have been observed before and the next will not occur until 11 December 2117.
See graphic: “Where to stand, what you’ll see”
“Every capable observatory in the world will have something trained on Venus,” says at the University of Arizona in Tucson. “This is a really rare opportunity – we won’t have another transit visible from Earth for another 105 years.”
Ironically, Venus’s hellishness – the very thing that turned space agencies off decades ago – is what astronomers are hoping to detect during this event. That’s because they will treat Venus like an extrasolar planet passing in front of an alien star and will try to detect the composition of its atmosphere as it passes in front of the sun. They want to know if what they see matches up with observations taken by spacecraft that have visited the planet. “There’s a knowledge about Venus’s atmosphere, but what that means in terms of observations – we don’t really know,” says Schneider.
“Venus’s hellishness – the thing that turned us off the planet – is what we are hoping to see this time round”
Will the picture from Earth reveal Venus to be the life-hostile world we know it to be? Astronomers are anxious to find out because they do not want to be fooled in their quest to track down habitable, Earth-like worlds around other stars. “There’s a feeling that if there’s a planet in an Earth-like orbit with an Earth-like size, somehow it’s going to be Earth-like,” says at the University of California at Santa Cruz. “Venus dramatically proves that isn’t the case.”
This year’s transit is not the first in recent memory. The alignments occur in pairs eight years apart, and there was a transit in 2004. Many astronomers observed it, but mostly just for fun. “We knew to expect some beautiful moments,” says of the Paris Observatory in France, “but there was little anticipation of how much the event would actually bring to planetary science.”
Things are different this time around. Widemann and others want to wring out as much science as possible and their interest is fuelled in large part by the explosion of exoplanet discoveries made since the last Venus transit. Back then, only three of the 120 or so exoplanets discovered at that time had been observed to dim the light of their stars in transits. Now, thanks in large part to NASA’s Kepler telescope, which launched in 2009 specifically to find transiting exoplanets, more than have been found transiting their host stars.
Moon mirror
Researchers did glean some science from the 2004 Venus transit. Using two NASA satellites designed to study the sun, Schneider and his colleagues found that the total light from our star dimmed by 0.1 per cent, exactly as expected for a planet Venus’s size ().
Pascal Hedelt of the , France, led a team that went a step further. They pointed the Vacuum Tower Telescope in Tenerife, Spain, at Venus’s disc during the transit and looked for the spectral signature of the sun’s light shining through the planet’s atmosphere. They . We already knew it was there from studying the spectrum of sunlight reflected off the planet itself, but Hedelt’s team proved that such techniques might help exoplanet hunters better understand their quarries.
Such an atmospheric observation is much easier than anything that can be done with super-remote exoplanets, though. Transiting exoplanets are too far from Earth to be seen as dark silhouettes against the bright face of their host stars – they merely register as dips in the overall brightness of their stars. “With extrasolar planets, you’re just getting the light hitting the detector. It’s not an image,” says Laughlin. As a result, astronomers cannot simply zoom in on the transiting planet’s disc. Instead, they must compare the spectrum of all the light from a host star before and during an unseen transit to see the overall change due to the planet’s atmosphere.
This measurement has been impossible for most known exoplanets, since it requires a very bright star to act as a backlight and a planet large enough to have an atmosphere that filters that light detectably. The stars that Kepler observes are too faint to be used for this purpose, but the Hubble and Spitzer space telescopes have been able to measure the spectral signatures of the atmospheres around a handful of exoplanets bigger than Earth.
Future telescopes, including the James Webb Space Telescope scheduled for launch in 2018, will be more sensitive to detecting exo-atmospheres. But the measurement is extremely difficult, as most wavelengths of light pass through only the part of the atmosphere above any clouds, leaving a very thin ring of transmitted light whose spectral signature must be picked out from the overwhelming glare of the star’s light.
Schneider and his colleagues were geared up to study Venus’s atmosphere during the 2004 transit. They realised that if they were to learn anything about future exoplanet transits from the measurement, they had to make the event slightly more lo-fi, by somehow blurring the observations so that Venus was invisible. Their trick was to use the surface of the moon as a mirror to observe the transit, so they would not see Venus’s shadow directly but instead would measure the spectrum of sunlight reflected from the moon before and during the event. Sadly, atmospheric conditions above Arizona, where their telescope was, did not cooperate. “The sky was clear but there was atmospheric turbidity,” Schneider says. “We didn’t have enough sensitivity to detect the transit.”
Nothing will get in the way this time around. To avoid Earth’s bubbling atmosphere, a team of astronomers is planning to observe the moon using the Hubble Space Telescope in the hope of picking out the spectral signature of Venus’s atmosphere.
of the Paris Institute of Astrophysics in France, who is a member of the team, has reason to believe they will succeed. In 2008, he and his colleagues used the moon to measure the spectrum of the Earth’s atmosphere when our planet passed between the moon and the sun during a lunar eclipse. They were able to measure several key gases, most notably molecular oxygen and ozone (). These molecules can be created by photochemical reactions, but also by life. “The amount of oxygen is so large in the case of the Earth that it is difficult to come up with a way for chemistry to do that,” says Vidal-Madjar. “If you don’t see any oxygen, you can argue the situation is not very good for life.”
Venus, too, has an ozone layer that lies high above the planet’s dense clouds, so it might be detected in this week’s transit (). Yet we know the amount – about 1 per cent of Earth’s ozone stores – is too small to suggest life. If transit measurements of exoplanets show a large amount of CO2 and a small amount of oxygen or ozone, this would suggest that the transiting planet is more like Mars or Venus than present-day Earth, says Widemann. Curiously, though, such a signature would also have fit the Earth 3.5 billion to 4 billion years ago – before most of our planet’s CO2 had dissolved within the oceans and long before photosynthetic microbial life had a chance to produce oxygen and ozone.
Given that range of possibilities, how much can we actually infer about exoplanets’ potential habitability from atmospheric measurements? “Not much,” concedes Vidal-Madjar. “But it’s a start. Just seeing CO2 can be proof of an early Earth or a Venus-like planet. The idea is to have a first-step observation and to think about what observations should be done next,” he says.
The transit will not only benefit exoplanet studies – Venus itself will shed some of its mystery. In past transits, when the planet started to enter or leave the sun’s disc, some observers noticed a faint arc of light appear on the side of the planet that did not lie in front of the sun. Called the , the arc is produced by sunlight refracting through the layers of Venus’s atmosphere lying above its thick cloud deck. From its appearance, of the Côte d’Azur Observatory in Nice, France, and his colleagues were able to calculate that it was produced between 80 and 120 kilometres above Venus’s surface (). Yet because they had not set out specifically to probe the aureole during the 2004 transit, they did not learn much more.
This time around, the team is poised to study the aureole in more detail, measuring small-scale changes in average temperature throughout this region. Such changes signal waves in Venus’s atmosphere that might help explain its strange dynamics. In the cloud layer, winds whip round 60 times faster than the planet itself spins. Yet they slow down higher up and change direction completely at an altitude of 120 kilometres. “We still don’t entirely understand what maintains the super-rotation and how the two wind regimes connect,” says Widemann.
Variations in atmospheric temperature and pressure have been spotted by the European Space Agency’s Venus Express probe, which went into orbit around the planet in 2006. But these have so far failed to explain the mystery, perhaps because Venus Express only measures the atmosphere at one place at a time. Widemann likens it to observing the weather above Scotland one day and Wales the next. If the weather is seen to be different at the two sites, it is impossible to say whether the conditions are the same at both places and the measured change was down to differences in the timing of the observations, or whether the weather simply varies by latitude.
See gallery: “Probe hints Venus once had oceans and plate tectonics“
The transit should shed light on how these changes relate to each other, since all latitudes of the atmosphere are illuminated in the aureole at the same time. “We will have one single measurement of the temperature field from one pole to the other,” says Widemann. “This is our only chance to do that.”
Other teams plan to use the close approach between Earth and Venus at around the time of the transit to ping radar signals off it from the Arecibo Observatory radio telescope in Puerto Rico. Radar pierces the planet’s obscuring clouds to reach its surface, and a team led by of the Smithsonian Institution in Washington DC want to compare the new readings with those taken from Arecibo in 1988. The technique could pick up changes in the roughness of the surface of only a few centimetres, which would be a . “We might see radar echo changes from a rugged lava flow that covered smoother plains, or from a smooth lava flow that covered earlier rugged material,” says Campbell. Venus’s surface is largely covered by lava that solidified less than 700 million years ago. The planet is expected to have begun its life with about the same amount of internal heat as the still-volcanic Earth, but no definitive signs of recent volcanism have yet been seen. Finding such signs would help answer major questions about how Venus loses its internal heat over time, says Campbell.
No matter what observations they are planning, astronomers are feeling the urgency of this transit. “There is some pressure,” says Widemann. He plans to observe the transit from Hokkaido, Japan, and will be prepared to move sites if bad weather rolls in. But he hopes to avoid the fate of his French countryman Guillaume Le Gentil, who spent more than 11 years away from home in a doomed attempt to observe the Venus transits of 1761 and 1769.
Le Gentil returned to find his relatives had tried to declare him dead and were about to divide up his estate, and his coveted place in the French Academy of Sciences had been given to someone else. “I don’t want to go that way,” says Widemann. “With good airport connections, hopefully that won’t happen this time.”
How to safely observe the transit
Never look at the sun directly. Instead, use eclipse shades, which are inexpensive and resemble 3D glasses but use special filters that safely block most of the sun’s light. Shade 14 welding glass is . Or point the main lens of a small telescope or binoculars at the sun – again, without looking through them at the sun – and project the image from the other end onto a white surface.
