The hunt is on for exo-Earths
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 13, 2008

Observations of the Earth by Venus Express, and supercomputer simulations of dusty discs around Sunlike stars may provide new clues in the quest to detect Earth-sized exoplanets, a goal that could be realised within the next 15 years.

Performing observations of the Earth to determine its habitability may seem like an odd thing to do when we know that it has an impressive inventory of life, but measurements of our enriched planet from afar can give important clues in how to detect life on other distant worlds. Since Earth spans less than a pixel across in Venus Express cameras, that is, as a single dot with no visible surface details, the observations mimic the challenges that planet hunters will face when searching for Earth-sized worlds around other stars.

"We want to know what can we discern about the Earth's habitability based on such observations,” says David Grinspoon, a Venus Express Interdisciplinary Scientist from the Denver Museum of Nature and Science, Colorado. “Whatever we learn about Earth, we can then apply to the study of other worlds."

This image composite shows the signatures of water (H2O) and oxygen (O2) detected by the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS) on board ESA’s Venus Express at visible and near-infrared wavelengths. Each curve corresponds to the Earth showing a different face to Venus Express (see the simulated Earth images at the top), and at different distances. The light detected by VIRTIS was mainly reflected by clouds. The spectra show the variations in results depending on cloud cover, location of glaciers and position of oceans in the field of view, and can give insight into the planet's rotation around its parent star. Image: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA (Earth views: Solar System Simulator JPL-NASA).

The images cover both visible and near-infrared regions of the spectrum and can be split into spectra to help identify the molecules of Earth’s atmosphere. But the data shows that looking for water and molecular oxygen is not enough, since both signatures are present in Earth and Venus measurements alike.

Scientists are now seeking alternative ways to identify the key signatures of a life-bearing planet, such as the subtle ‘red edge’ signal caused by photosynthetic life - the bright infrared fingerprint of green plants. The presence of oceans may also be revealed in high resolution spectra, and the Venus Express team will compare spectra of the Earth's oceans with those taken when the continents are facing the spacecraft.

The observations may not tell us anything new about our home planet, but they are teaching important lessons in how to resolve images of far-off worlds. And while it may be some time before imaging capabilities allow astronomers to directly image Earthlike planets around other stars, supercomputer simulations may provide the signposts that could point astronomers in the right direction.

Indeed, a study conducted by Christopher Stark of the University of Maryland and Marc Kuchner of NASA’s Goddard Space Flight Center show that patterns in dusty discs around Sunlike stars created by planets nearly as small as Mars could be detected by future exo-Earth-hunting telescopes.

"It may be a while before we can directly image Earthlike planets around other stars but, before then, we'll be able to detect the ornate and beautiful rings they carve in interplanetary dust," says Stark.

Click for animation. A planet twice Earth's mass forms a ringed dust structure in this simulation. Enhanced dust density leads and trails the planet and causes periodic brightenings. Image: NASA/Christopher Stark, GSFC.

The scientists modelled the response of 25,000 dust particles to the presence of a single planet, ranging from the mass of Mars to five times the mass of Earth, orbiting a Sunlike star. In total the team ran 120 different simulations with varying dust particle sizes, planetary mass and orbital distance to study the density, brightness, heat signature and shapes of the ring structures created in the dust. The computer models also account for the dust's response to gravity and to the star's light, which exerts a slight drag on small particles that makes them lose orbital energy and drift closer to the star.

"The particles spiral in toward the star, get trapped in one resonance, fall out of it, spiral in some more, become trapped in another resonance, and so on,” says Kuchner. A resonance occurs whenever a particle's orbital period is a small-number ratio of the planet's. For example, if a dust particle makes three orbits around its star every time the planet completes one, the particle will repeatedly feel an extra gravitational tug at the same point in its orbit. Stark and Kuchner show that this extra nudge can offset the drag force from starlight, allowing the dust to settle into subtle ringlike structures.

Some scientists note that the presence of large amounts of dust could present an obstacle to directly imaging Earthlike planets,
but the models created by Stark and Kuchner will give astronomers a preview of dust structures that signal the presence of otherwise hidden worlds.

This image shows the results of a face-on simulation for an Earth-mass planet. Colours indicate density with purple revealing regions with the lowest density of dust and red the highest. Patterns in dust observed around stars could point astronomers to otherwise hidden planets. Image: ASA/Christopher Stark, GSFC.

According to The Exoplanet Task Force of the Astronomy and Astrophysics Advisory Committee, the first detection and characterisation of Earthlike planets around other stars could occur within a time frame of just 15 years. The Task Force report demands a two-pronged attack on the search for Earthlike planets, with complementary observations from the ground and from space-based telescopes. The first attack will come from telescopes already in existence or under development to search for and characterize Earth-sized planets around the smallest stars, although these stars will be much different from our own Sun. The second prong envisages the development of more advanced techniques to search for stars like the Sun that may be harbouring Earthlike planets, such as a spaceborne astrometric telescope that can measure the tiny perturbations inflicted on the star's apparent motion by the orbiting Earth-mass planet. Once planet hunters have the ‘addresses’ of such stars and know where to look, an imaging system in space could determine the atmospheric composition of such bodies.  

Discovering a true Earth analogue, that is, a planet of one Earth mass or one Earth radius orbiting a Sunlike star at a distance similar to the Earth-Sun separation would answer one of the great questions of modern science: are planets like Earth common or rare?

With CNES–ESA's COROT spacecraft currently in orbit, NASA's Kepler mission and the James Webb Space Telescope scheduled for launch in 2009 and 2013 respectively, and the proposed Terrestrial Planet Finder, the goal of discovering Earth-sized worlds in Earthlike orbits around other stars is getting closer and closer.