Rock, Scissors, Paper and Water

The classic game of "Rock, Scissors, and Paper" includes a circular hierarchy of winning plays. The rock crushes scissors, scissors cut paper, and paper covers the rock. Scientists seeking life elsewhere have a similar hierarchy to classify the habitability of distant planets.

Their planet search will try to find rock, iron and organic stuff. This hypothetical earth needs a magnetic field, and thus an iron-rocky core, or it will quickly lose its atmosphere. It also needs organic material, much like the tree pulp of pressed paper, which elementally includes carbon. In the next few years, a powerful new planet detector, called Corot, will be launched by the European Space Agency, and offers the first telescopic ‘scissors’ in orbit. Corot will act like scissors because it depends on a careful angle, or line of sight, between a planet and the star it orbits. But to finish their hierarchy, a new play must be added to the game: Water.

On such a detected planet, liquid water makes their search about finding life. Water may trump all other elements in the life detection game. While water erodes rock, it also dissolves paper or organics. That dissolution makes possible biological molecules, cell membranes, and nutrient metabolism.

Indeed one of NASA‘s guiding policies in the search for life elsewhere is to "follow the water." While water is fairly common in the universe, found everywhere from vast interstellar dust clouds to the orange-red fields of Mars, most of this water is in the form of ice. In 1998, NASA‘s Associate Administrator Wesley Huntress, Jr., stated, "Wherever liquid water and chemical energy are found, there is life. There is no exception."

Remarkable frozen texture on Jupiter’s moon, Europa.
Credit: NASA/JPL

At a recent ESA co-sponsored ‘Towards Other Earths’ conference, nearly 250 of the world’s leading experts in planet detection discussed the strategy for finding Earth-like worlds. Alain Léger and colleagues of the Institut d’Astrophysique Spatiale, France, described a new class of planets that could be awaiting discovery: ‘waterworlds’.

According to Léger and his colleagues, these waterworlds would contain about six times the mass of Earth, in a sphere twice as wide as our planet. They would possess atmospheres and orbit their parent star at roughly the same distance that the Earth is from the Sun. An ocean of water entirely covers each world and extends over 25 times deeper than the average depth of the oceans on Earth.

Seeing Deep Into the Deep

According to calculations, the internal structure of this waterworld would consist of a metallic core with a radius of about 4000 kilometers (2400 miles). Then there would be a rocky mantle region extending to a height of 3500 kilometers above the core’s surface, covered by a second mantle made of ice up to 5000 kilometers thick. Finally, an ocean blankets the entire world to a depth of 100 kilometers, with an atmosphere on top of this.

All this speculation on size is important, when deciding what kind of telescope to build. With twice the radius of the Earth, the class or waterworlds will be easily spotted by the Eddington spacecraft, which is designed to detect planets down to half the size of the Earth. "A waterworld passing in front of a star, somewhat cooler than the Sun, will cause a dimming in the stellar light by almost one part in a thousand. That’s almost ten times larger than the smallest variation Eddington is designed to detect. So, waterworlds – if they exist – will be a very easy catch for Eddington," says Fabio Favata, ESA’s Eddington Project Scientist.

Planet in Transit

The CNES/ESA mission Corot, which is a smaller, precursor mission to Eddington, is due for launch around 2005, and Corot may also be just able to glimpse waterworlds, if they are close enough to their parent stars. The Corot spacecraft consists of a 30 cm telescope with an array of charge-coupled devices, or CCD’s, for sensitive light detection. It will monitor the light-curves of well chosen stars. The overall potential of COROT is to detect several tens of Earth-sized planets.

By combining the high sensitivity of space telescopes with the sharply detailed pictures from an interferometer, TPF will be able to reduce the glare of parent stars to see planetary systems as far away as 50 light years.
Credit: NASA

To achieve this near-term goal, the mission relies on identifying a planet close to its parent star, then timing a snapshot along a narrow line of sight when the planet eclipses the starlight viewed from the orbiting telescope. Called the transit method, the idea of detecting planets by their eclipsing power was originally proposed fifty years ago [by O. Struve].

While more than a 100 giant planets have been discovered total, most successful surveys to date have not looked for transits, but instead looked for the gravitational wobble that these giants induce in their parent star. That ‘radial velocity’ method, however, has fundamental limits that will not allow the discovery of planets smaller than about 10 Earth masses. That limit is one reason planetary transit detection has intrigued scientists enough to add Corot as a near-term opportunity and thus enhance their planetary encyclopedia with more Earth-like candidates. Three exoplanets have been discovered so far using the transit method.

But even with Corot’s ability to see Earth-sized planets, its alignment involves a narrow angle for success–indeed, a scissor-like action is needed to cut up the night sky into precise survey sectors. The chances that a particular exoplanet passes in front of the disk of its central star as seen from the Earth are small. As Hans Deeg, of the Instituto de Astrofísica de Canarias, wrote, "In order to produce transits, it is of course necessary that a planet-star system orbits in a plane that is within a small angle to the line of sight."

"For the Earth-Sun system, this probability is 0.47%," continues Deeg. "Considering that Venus is another Earth-like planet with similar size, and its probability for alignment is 0.65%, the total probability for an external observer to detect any Earth-like planet around the Sun is about 1%. The probability to detect short periodic planets is of course larger, and in the case of Hot Giant planets it reaches over 5%. ..[but] giant stars are too big and photometrically unstable to produced observable planetary transits. Thus, not more than one in 200 field stars can be expected to show transits of Earth-like planets – this if Solar Systems equivalents are very common. Consequently, thousands of stars need to be observed, causing the need for wide-field photometric cameras transit surveys."

For this reason, Corot is scheduled to look at 12,000 chosen stars. With a launch date planned for 2007, the four-year mission duration of NASA’s Kepler project will expand this survey to 100,000 stars, as will roughly ESA’s Eddington [50-100,000 stars].

What’s Next

For most planet finders, the real challenge is to identify faint planets in the glare of their much brighter parent stars. To overcome the distortion of how our own atmosphere may further obscure this detection, both large land-based telescopes and space missions will likely combine in the future to complete the picture. In addition to the space-based, planet surveys- Corot, Eddington and Kepler -later missions will attempt to refine the criteria of the search itself. Novel ways to detect water and other ingredients will be needed for a definitive signal that the game of life is being played elsewhere. For instance, by combining the high sensitivity of space telescopes with the sharply detailed pictures from an interferometer, NASA’s Terrestrial Planet Finder, or TPF, will be able to reduce the glare of parent stars to see planetary systems as far away as 50 light years.

One way to know if anything lives on a waterworld will be to study them with ESA’s habitable-planet-finding mission, Darwin. When it launches in around 2014, this flotilla of spacecraft will look for tell-tale signs of life in the atmospheres of any planets, including waterworlds.

NASA is developing the Terrestrial Planet Finder (TPF) project, part of the Jet Propulsion Laboratory Navigator Program, and ESA is developing its DARWIN project.

Related Web Pages

Search for Life in the Universe: Part I
Water Worlds
Edward Weiller: Are We Alone?
Chris Chyba: The Search for Life
Water: The Molecule of Life
Conditions for Life: Water
Water: The Hub of Life
Table Talk
Planet Quest (JPL)
Kepler Mission
Eddington Mission
Darwin Mission
Herschel Mission
Space Interferometry Mission