The light from alien worlds can tell you a lot about the place—if you know what to look for. But to assess whether or not a distant planet has living organisms, you need to model many aspects. That’s where the Virtual Planetary Laboratory (VPL) comes in. The project aims high: simulating entire worlds so that telescopes here on Earth can decode the signals from directly-imaged exoplanets. Running since 2001, the VPL’s work is becoming ever-more important.
Astronomers have been able to capture light from Jupiter-sized behemoths around other stars and even a few Neptune-sized and smaller worlds. But in the coming years, new observatories will be able to directly image rocky planets in their parent star’s habitable zones, analyzing the composition of their atmospheres and even peering at their surfaces. One of the biggest question in modern-day astronomy—whether or not life exists elsewhere in the cosmos—could be answered in our lifetimes.
Earth, The Pale Blue Dot. Credit: NASA
The word interdisciplinary barely suffices to describe the VPL team. Astronomers, chemists, geologists, climate modelers, oceanographers, microbiologists, and experts on stellar and planetary evolution are all necessary to accomplish the project’s goals. “We talk about everything from oceans and land to the interior, atmosphere, and magnetosphere; as well as how the planet interacts with the star, and even how the star’s gravity modifies the planet’s orbit,” says Victoria Meadows of the University of Washington in Seattle, the VPL’s principal investigator. “One of my catchphrases is: ‘It takes a village to model a planet.’”
The end product is deceptively simple; simulated light curves and spectra that serve as potential examples for what researchers could one day observe with their telescopes. Atmospheric spectra contain the fingerprints of different chemicals while starlight reflected from a planet can provide clues about the absence or presence of oceans, clouds, ice, or surface vegetation. When data from dozens of distant worlds starts pouring in over the next couple decades, the VPL team will be ready to explain their secrets.
The obvious first place to start is with our own planet, the only place in the universe we know of with living organisms. VPL researchers have produced a detailed model of what the Earth would look like from afar—similar to the famous Pale Blue Dot image from the Voyager missions—that could allow scientists to identify a potential planetary twin. But that doesn’t cover the full story of our world. “Let’s go back in time,” says Meadows. “The Earth has been a whole series of habitable planets throughout its history, and can we think of different types of metabolisms that dominated Earth’s environment in the past.”
Combining geological and climatological models with inputs from biological and geological data, the VPL team have simulated the early Earth at different points. For example, between 4 and 2.5 billion years ago our planet was an alien place, with bacteria belching out tons of methane into the atmosphere. The methane would interact with ultraviolet light from the early sun to create a hydrocarbon haze over our world, forming clouds and providing a shield to solar radiation prior to the rise of ozone. Seen from a distance, this Pale Orange Dot might look similar to Saturn’s moon Titan.
The “Pale Orange Dot” that is Saturn’s hazy moon, Titan. Credit: NASA.
The VPL team has also explored very different worlds from our own, and what their composition could tell us about the prospects of life. Recent models have replicated rocky planets around red M dwarf stars, which are smaller and cooler than our sun but are the most common stars in the galaxy. When they’re young, M dwarfs are super-luminous, meaning they produce excessive amounts of light. Any terrestrial planet orbiting around them will be subject to lots of radiation for its first billion or so years.
“We’re exploring the different scenarios for how we might be fooled.”
Were such worlds to have oceans at the beginning of their lives, the water could all be boiled into the atmosphere, where radiation would split it into hydrogen and oxygen. The light hydrogen would float off into space, but the oxygen could stick around; in fact, these places might have hundreds of times the oxygen of our own world but would be much drier than Earth. Such findings imply that oxygen alone is not enough to indicate a habitable planet—too much of a good thing might actually point to the opposite.
An artist’s imagination of hydrocarbon pools, icy and rocky terrain on the surface of Saturn’s largest moon Titan. Credit: Steven Hobbs
Many upcoming missions will look at M dwarfs, such as the Transiting Exoplanet Survey Satellite (TESS), a successor to the Kepler space telescope that will launch next year and have the 1,000 closest red dwarfs among its target. Hot on its heels will be the James Webb Space Telescope (JWST), which could peer at any nearby planets TESS finds and take spectra of their atmospheres. The VPL data will allow researchers to discriminate between oxygen-rich potentially habitable worlds and oxygen-far-too-rich uninhabitable ones. “It’s kind of forewarned is forearmed,” says Meadows. “We’re exploring the different scenarios for how we might be fooled.”
Atmospheric chemicals are not the only way to assess planetary habitability. Future telescopes might one day look for glints of light coming from a surface ocean. Seen from Earth, a planet orbiting its host star can go through different phases, much like the moon changing phases in the night sky. Liquid on the surface of an exoplanet could act like a mirror, reflecting starlight and making it much brighter in certain phases than a dry planet. “This might be one of the most unambiguous signals we’ll see,” says Meadows.
A surface ocean is likely to bump a planet up a few notches in its interest to scientists. Researchers can certainly imagine organisms that live deep underground or beneath an ice shell but “when we’re trying to detect life from 10 parsecs we want it on the surface, ideally waving at us,” says Meadows. Another signal the VPL team has explored is a regular seasonal change in a planet’s reflectivity or atmospheric composition. The annual cycle of plants on Earth—growing in the spring and summer then dying and decomposing in the fall and winter—produces a sinusoidal carbon dioxide pattern that could be spotted from far away.
A glint of sunlight reflected off the mirror-like surface of a hydrocarbon lake on Saturn’s moon Titan. Credit: NASA/JPL/University of Arizona/DLR
Of course, James Webb is not specifically designed for exoplanet searches and is likely to only provide the first clues to life elsewhere in the galaxy. In the next decade, a new generation of enormous ground-based telescopes, such as the Thirty Meter Telescope, Giant Magellan Telescope, and European Extremely Large Telescope, will allow researchers to directly image more exoplanets and help the field really take off. Farther afield, astronomers are hoping to launch a new space-based telescope that could add many Pale Colorful Dots to the current meager catalog. Once those missions get going, the VPL will be well placed to interpret their findings. “That data will be challenging, but we will have already done a lot of the legwork,” says Meadows. “And I’m sure there will be plenty of surprises to learn from.”