[/caption]If you are an astronomer looking for a habitable exoplanet orbiting a far-off star, what do you look for? We know from personal experience that we need oxygen and water to live on Earth, so this is a good place to start; look for exoplanets with the spectroscopic signature of O2 and H2O. But this isn’t enough. Venus has oxygen and water in its atmosphere too, so if we only used these two indicators as a measure for habitability, we would be sorely disappointed to find a water and oxygen-rich Venus-like world which has little chance of supporting life (as we know it).
In an effort to understand what a “habitable planet” looks like from afar, European Space Agency (ESA) scientists have decided to do a bit of retrospective astronomy. Venus Express, currently in orbit around Venus, is being used to look back at the blue dot we call home to help us understand what a real habitable planet looks like…
Venus Express (sister ship of ESA’s Mars Express) was launched in November 2005 to begin its seven month journey to Venus. As the spacecraft left Earth orbit, it turned around to take a picture of the blue globe with its Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), but the significance of this quick observation wasn’t realised until a year after Venus Express had entered Venusian orbit. Could the robotic craft be used to watch the Earth from afar?
Giuseppe Piccioni, Venus Express VIRTIS Co-Principal Investigator, in Italy, has been heading a sustained campaign of Earth observations using the VIRTIS instrument orbiting a planet 0.3 AU closer to the Sun. Although Venus has often been referred to as “Earth’s sister planet” the difference couldn’t be more stark. With atmospheric pressures some hundred times that of the Earth, with a choking cocktail of poisonous gases and high surface temperatures, Venus is hardly conducive for life. Earth, on the other hand, has a bountiful ecosystem where life has thrived for over three billion years. However, Piccioni is aware that if viewed from a distance, both Earth and Venus contain some of the basic ingredients for life; how can we be sure distant exoplanets are more Earth-like or more Venus-like? After all, planet habitability doesn’t seem to depend on just oxygen and water.
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“We see water and molecular oxygen in Earth’s atmosphere, but Venus also shows these signatures. So looking at these molecules is not enough,” says Piccioni. So, in an attempt to seek out other forms of life, the Italian astronomer is looking toward Earth to pick out more subtle signals for the presence of life on alien worlds.
Venus Express can observe Earth about three times a month, and over the last two years, VIRTIS has captured 40 terrestrial images for analysis. The light captured from these Earth observations cover spectral wavelengths from visible through to near-infrared, but when viewed from Venus, the Earth appears only as a small dot, no bigger than a single pixel in Venus Express’ cameras. Far from being a hindrance, this small dot will help future exoplanet hunters.
Although there are no surface features, this small dot still holds a lot of information. By splitting the light observed into its component wavelengths, the composition of the terrestrial atmosphere can be analysed. Therefore, spectroscopic signals from plant life could be detected for example. “Green plants are bright in the near infrared,” said David Grinspoon, a Venus Express Interdisciplinary Scientist from the Denver Museum of Nature & Science, Colorado, who suggested the programme of sustained Earth observation. “We want to know what can we discern about the Earth’s habitability based on such observations. Whatever we learn about Earth, we can then apply to the study of other worlds,” he added.
Exoplanet hunters are finding more and more alien worlds orbiting stars many light years away, it is only a matter of time before we have the technological ability to image the one-pixel spot of an Earth-like world. By understanding how our habitable planet looks from Venus, we can begin to understand whether these exoplanets are indeed “Earth-like” in every sense of the word…
Here’s an article about the famous blue dot image of Earth.