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Extrasolar planets

Water world larger than Earth

Nature volume 462, pages 853854 (17 December 2009) | Download Citation

The hunt for Earth-like worlds has taken a major step forward with the discovery of a planet only 2.7 times larger than Earth. Its mass and size are just as theorists would expect for a water-rich super-Earth.

Momentous breakthroughs in science often come unexpectedly and serendipitously, requiring decades of patience. Only rarely does a long-sought scientific frontier loom so prominently just beyond the horizon that the next generation of instruments seems sure to reach it. A tantalizing case for such a breakthrough is presented by Charbonneau et al.1 on page 891 of this issue. They provide the most watertight evidence so far for a planet that is something like our own Earth, outside our Solar System.

Charbonneau and his co-workers developed a simple and forward-looking planet-hunting technique. They installed a suite of eight amateur-sized telescopes (with 40-cm-diameter mirrors), each with a sensitive charge-coupled-device light detector that measures the near-infrared brightness (wavelengths of about 700–900 nanometres) of a star. Any star whose brightness dims for about an hour, and repeats that dimming like clockwork over the course of days and weeks, is probably doing so because an orbiting planet is crossing briefly in front of it, blocking a fraction of the star's light. The amount of dimming directly indicates the size of the planet relative to that of the star. From a large sample of nearby stars2, Charbonneau et al. have smartly chosen the 2,000 of smallest radii, so that near-Earth-sized planets would block at least 1% of a star's light, rendering such worlds detectable.

Charbonneau's team1 has found that the small, faint star GJ 1214 undergoes repeated dimming of 1.3% for 52 minutes every 1.6 days. The only plausible interpretation is that a planet orbits the star with an orbital period of 1.6 days and that it has a radius that is 12% that of the star. Good estimates of the star's radius (21% that of the Sun) put the planet's radius at only 2.7 Earth radii. Such a small planet orbiting a star other than the Sun is an extraordinary find. With the tools currently available, only one other extrasolar planet has been reported that is thought to be close in size to Earth, namely CoRoT-7b, at 1.7 Earth radii. The new planet, which is only about 13 parsecs away, is named GJ 1214b. Importantly, it pulls gravitationally on its host star, causing the star to move with a speed of 12 m s−1, which the team has detected through measurements of wavelength shifts in the star's light (the Doppler effect). The planet's inferred mass is a mere 6.6 Earth masses, which, when combined with its radius, leads to a density of 1.9 g cm−3. By contrast, Earth's average density is much higher, at 5.5 g cm−3. Because water has a low density of about 1 g cm−3, the chemical composition of the new planet is probably some admixture of rock and water, with perhaps a small atmosphere of hydrogen and helium.

Could this planet have a solid surface suitable for hosting organic-rich ponds and lakes? Some astronomical background offers a good guess. The protoplanetary disks of dust and gas swirling around young stars are the sites of planet formation. The disks are made up of the same admixtures of H and He gas, carbon, nitrogen and oxygen compounds, and iron and nickel metals found in nearly all of the stars in our Galaxy, including the Sun. Solid dust particles made of Fe, Ni, silicates and ices stick together and grow into ever larger planetesimals, forming the basic cores of all planets.

The relative amounts of these solid constituents vary only modestly among different protoplanetary disks, for two reasons. First, the abundances of the atomic elements are nearly the same, within factors of two, from star to star, with C, N, O, silicon, magnesium and Fe being the building blocks of the solid material. Second, the highly negative Gibbs free energy of carbon monoxide and silicates locks up as much oxygen as the limiting reagents — C and Si — permit, leaving plenty of oxygen to form water ice, despite its higher Gibbs free energy. Thus, silicates and water ice dominate the mass budget of the solid material in the cold regions of protoplanetary disks, along with the Fe and Ni dust grains.

That solid material forms the building blocks of large planets such as Saturn and Neptune, and perhaps smaller planets as well, such as the new one1. But the density of 1.9 g cm−3 for this new planet imposes a constraint on the relative amounts of each constituent. To keep the planet's density that low requires that it contains large amounts of water. If the planet were pure Fe and silicates, its density would be similar to Earth's. It must contain a huge amount of water, roughly 50% by mass.

The wild card is the amount of H and He gas in the atmosphere. Spooning additional H and He (of low density) onto a planet makes its density lower, which can be compensated for by increasing the amount of Fe in the core to bring the overall density to that measured, 1.9 g cm−3. But the planet-building environment is unlikely to spawn planets composed of mostly Fe and H/He, but very little water. Any planet that contains Fe, rock and H/He would have also retained correspondingly large amounts of water. Thus, it is likely that this new world has nearly 50% of its mass in water surrounding an Fe/Ni core and a silicate mantle (Fig. 1). It probably has an extraordinarily deep ocean, which would be liquid given its equilibrium surface temperature of some 190 °C due to heating from the host star. A sauna-like steam atmosphere is possible, with slow photolytic and hydrodynamic loss of that atmosphere caused by ultraviolet-light irradiation. A thin H/He outer atmosphere is also possible.

Figure 1: A water-rich super-Earth?
Figure 1

The newly discovered1 extrasolar planet, GJ 1214b, probably contains a huge amount of water, surrounding an inner core of iron and nickel, and an outer mantle of silicate rock, and may have a small atmosphere of hydrogen and helium. Only 2.7 times larger than Earth, and just 13 parsecs away, this super-Earth brings astronomers closer to discovering Earth-like planets.

And so comes the profound anthropocentric question. If this planet is 50% water, is it really kin of our Earth? Or did it form in a manner similar to that of Saturn or Neptune, with a rocky core that acquired large amounts of ices and gas gravitationally? By contrast, Earth has only 0.06% water, and very little H and He gas, having formed in a dry environment. This new planet is close to Earth in size, but perhaps not next of kin.

Nonetheless, Charbonneau's team1 has highlighted a promising future for the discovery of Earth-like worlds. Their efforts are just beginning, with smaller and rockier planets yet to be found. Meanwhile, precise Doppler measurements may reveal the gravitational wobbles of stars caused by Earth-like planets in tight orbits. Most promising is NASA's Kepler mission. Launched in March 2009, Kepler is monitoring 100,000 stars and is able to detect dimming as small as one part in ten thousand of their normal brightness, rendering truly Earth-sized planets easily detectable. And someday, great space-borne interferometers (such as NASA's Space Interferometry Mission) and enormous cameras will be launched, able to detect, image and spectroscopically analyse the landscapes, oceans and atmospheres of nearby rocky planets. These techniques will surely answer the question Aristotle, Epicurus and Democritus posed 2,400 years ago regarding Earth's unique status in the Universe.

References

  1. 1.

    Nature 462, 891–894 (2009).

  2. 2.

    Astron. J. 130, 1680–1692 (2005).

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  1. Geoffrey Marcy is in the Department of Astronomy, University of California at Berkeley, Berkeley, California 94720, USA.  gmarcy@berkeley.edu

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