SPACE EXPLORATION

Exo-Earths—the search for worlds like our own

Focus is on finding planets with water and air at the right distance from a stable parent star.


Innumerable stars at the heart of the Omega Centauri globular structure, captured by the Hubble Space Telescope.

There was Bernard de Fontenelle (1657–1757) permanent secretary of the Paris Académie des Sciences, who in 1686 published his enormously popular Entretiens sur la pluralité des mondes (Conversations on the plurality of worlds) arguing for the existence of other worlds like Earth.

There was the Anglo-German William Herschell (1738–1822), who before his discovery of Uranus in 1781 first made his mark among astronomers by arguing that the Moon is inhabited by intelligent beings.

German philosopher Immanuel Kant (1724–1804) and English-born American revolutionary Thomas Paine (1736-1809) also argued for the existence of other “Earths”—and the list goes on.

Others, to be sure, were skeptical. Isaac Newton (1643–1727) believed that God created the universe specifically for Man, and the philosopher Georg Wilhelm Friedrich Hegel and his followers also rejected the notion of other Earth­like worlds.

So did the English scientist and philosopher William Whewell (1794–1866), who was concerned about the implications of the notion of many worlds for Christianity, and argued that scientific data simply did not support the idea. But gradually, over the course of the past 150 years, the case for the existence of other Earths became stronger.

As the true size of the universe and the number of stars within it came to be known, and as the Sun was downgraded from the center of the universe to an average star in the spiral arm of an average galaxy, it became increasingly hard to argue that Earth was unique.

By the second half of the 20th century, most scientists came to believe that worlds much like ours must exist out there in space. Finding such worlds, however, has proved extra­ordinarily difficult, and is one of the toughest challenges still facing astronomers today.

What makes an Earth?

Until the mid-1990s, the question of the existence of Earthlike worlds orbiting distant stars was wholly speculative. Although scientific theories predicted that planetary systems were common in the universe, no planet had ever been discovered orbiting a star other than our Sun.

But in 1995 a team of astronomers headed by Michel Mayor of the Geneva Observatory detected the first exoplanet—a gas giant orbiting the star 51 Pegasi. Since then more than 400 exoplanets have been detected, and the number is growing fast.

No planet like our own has as yet been discovered, but the discovery of an Earth-twin is likely only a matter of time. What used to be the subject of speculation and statistics is now an active field of astronomical research.

The search for exo-Earths is on. The first question confronting scientists searching for exo-Earths appears deceptively simple—What are we looking for? In other words, given a planet orbiting a faraway star, what characteristics would show that it is Earthlike?

The simple answer would be that we should go there and look, and decide whether this alien world is like our own. But interstellar flight, whether for humans or robots, is unlikely to materialize for a very long time, if ever.

For the foreseeable future we will have to limit ourselves to observations and measurements made from Earth itself or its immediate vicinity. This being the case, astronomers must decide what characteristics, detectable from light years away, are the telltale signs of an Earthlike planet.

The first and most critical feature that makes an exo-Earth is its mass. Most of the distant planets discovered to date are gas giants like Jupiter and Saturn, dozens or hundreds of times more massive than Earth.

This is only to be expected—the chief method used to date for detecting exoplanets is the radial velocity (RV) technique, which measures the regular shifts in a star’s spectrum as it rocks back and forth to the tug of an orbiting planet.

This method is very sensitive to the planet’s mass—the larger the planet’s mass the greater the star’s motion, and the stronger the RV signal. It follows that the planets most easily found would be the most massive.

But giants like Jupiter are poor candidates for astronomers searching for Earth analogs. Not only is their gravity crushing, but they are composed almost entirely of gas and have no recognizable “surface.” In contrast, exo-Earths must have hard rocky surfaces, and from what we know of planetary formation this means they must be far smaller than Jupiter.

At the same time, other “Earths” cannot be too small. Very low mass planets do not generate sufficient tectonic activity and do not hold on to their atmospheres—both essential features for a life-supporting planet.

Mars, for example, with a diameter 1/2 that of Earth and a mass just over 1/10, is too small to be a proper Earth analog. All of which is to say that exo-Earths can’t be too big or too small, but must be fairly close in mass to our home planet.

Another important criterion for an exo-Earth is that liquid water would be stable on its surface. Water is the key ingredient for all life as we know it, and no world that lacks liquid water would remotely resemble our own. Unfortunately, the locations where liquid water can exist in the universe are extremely limited.

In the vast cold stretches of interstellar space, water can only exist in the form of ice, whereas in close proximity to stars all water is heated and turned to vapor. Liquid water is only stable in a relatively narrow band around each star—close enough that the surface is not icy, but far enough that water is not vaporized.

This band is known as the habitable zone (HZ), and it is different for each star, depending on its size and temperature. Any exo-Earth would have to orbit precisely within the HZ band of its star. Other criteria also factor into deciding whether a planet can be considered Earthlike.

The planet’s orbit should be close to circular, maintaining a more or less constant distance from its star. If the planet’s orbit is overly eccentric, conditions on the surface will alternate between extremes of hot and cold, making it difficult for life to evolve and survive.

Such an orbit would also make it less likely that the planet’s path remains within the HZ. Furthermore, the exo-Earth’s home star must remain stable for billions of years at a stretch. Only such long-term stability will allow life to evolve and flourish without being subjected to extreme conditions.

This means that the star’s radiation must remain steady over time, and also that the star cannot be overly big—huge stars many times the mass of the Sun have much shorter life-spans, often lasting millions rather than billions of years. All of which boils down to this.

An exo-Earth must be a low-mass rocky planet with a circular orbit within the habitable zone of a stable small to medium star with low variability. There is no guarantee that a planet with all those characteristics will actually be another Earth—a water-rich world home to a vast menagerie of complex life—but these seem to be minimal requirements for an Earthlike planet.

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