The Life of Super-Earths (3 page)

The largest planets in our planetary system resemble the Sun in another important way—they have no solid surface or geography. From the top of the atmosphere that we see going down, it is all clouds and more clouds, getting denser and hotter as we sink deeper. Most of Jupiter's interior is hydrogen and helium under pressures a million times higher than we are used to on Earth. One reason why the pressure inside is so much higher is that the larger the planet the stronger it pulls itself together by its own gravity—you and I would weigh 2.4 times more on Jupiter. If we were to venture deeper inside the planet, like diving in the ocean, the pressure would become higher as well. No wonder, then, that things can get a bit out of hand inside Jupiter. The hydrogen gas turns into a liquid known as metallic hydrogen. It conducts electricity, which is why we call it that; otherwise the substance has the least bit of resemblance to the copper wire in your bedside lamp. Studying the properties of this exotic material is a challenge in a lab—it was produced on Earth about ten years ago. Today we know it sufficiently well to describe—in computer calculations—more or less confidently the interiors of Jupiter and Saturn, and consequently hot Jupiters like 51 Peg b as well.

Both Jupiter and Saturn have a small core (small for them, but enormous by Earth standards) made of elements heavier than hydrogen, helium, and neon. A core is typical of a planet, left over from its birth and formative years. For comparison, stars are born without cores and live long without them. As they age, stars grow a core, as lighter elements are fused into heavier ones, which simply pile up inside the star. Surprisingly, Saturn's core, with a mass of about fifteen planet Earths, is bigger than Jupiter's, at three to ten Earth masses. Or at least we think so. Jupiter is so much bigger, with so much metallic hydrogen, that the content of its core is difficult to determine. If its core indeed turns out to be smaller than Saturn's, that could have happened by birth, or it could have eroded slowly and gotten mixed into the upper layers. More importantly, both Jupiter and Saturn have a similar fivefold excess of heavy elements compared to the Sun in their core and mixed in throughout, revealing in no uncertain terms their planetary ancestry.

Uranus and Neptune are a different story. While Jupiter and Saturn, like the Sun, are mostly hydrogen and helium, Uranus and Neptune have only 10 percent of their mass in hydrogen and helium. The rest contains lots more oxygen, carbon, and nitrogen, in the form of frozen water, ammonia, and carbon dioxide. Although ten to twenty times less massive than Jupiter and Saturn, they are giants compared to Earth, and so they are known as the ice giants. Pluto is compositionally quite similar to Uranus and Neptune, but much smaller.

 

FIGURE 2.1
.
Proportions of the most abundant elements in the Universe today. This is the makeup of our Sun and most of our Milky Way Galaxy.

Much closer to the Sun is the province of the terrestrial planets, where Earth (a.k.a. Terra) rules in size and mass over Mercury, Venus, and Mars. Here hydrogen is almost gone—less than 0.1 percent by mass—and helium is virtually nonexistent.
²
The terrestrial planets are mostly oxygen, iron, and silicon, although iron predominates on Mercury. Most of the iron in these planets resides in central cores. During the planets' formative periods, iron (and a few other metals, such as nickel, that could not be part of the rocks) precipitated in large droplets in the center of the planets. The opposite is true of water: some is bound in rocks, but the rest, rather than sinking, stays at the surface. If the temperature and atmospheric pressure are right, a terrestrial planet will have liquid oceans.

An obvious question, given how different these planetary groups seem, is whether they could have come from the same “stock.” The modern Kant-Laplace model teaches that planets form from material left over from the making of the star, which consequently ought to have the same proportions of heavy and light elements. Images taken with the Hubble space telescope and the infrared Spitzer space telescope show that planet-forming disks are just 1 percent as massive as their stars, and that less than 2 percent of that mass is in all the elements heavier than hydrogen and helium. So why don't the small planets have any hydrogen or helium? Because of their mass. It takes the gravitational pull of a very large planetary seed to catch and keep those light gases. Smaller planets, such as Earth or Pluto, just can't hold them, and the intermediate-mass
ice giants formed so far out in the disk that they could only grow slowly. By the time they were ready to catch the hydrogen and helium, it had all but dissipated.

Now we can project the knowledge we have gained about these planets onto the planets discovered around other stars. We have seen among them Jupiters and Saturns, with small or large cores, and we have seen Neptunes as well. And we have seen more diversity than we ever imagined. As we hone our techniques to discover and study smaller planets, we are in for more surprises.

In the hierarchy of structures and objects in the Universe, planets occupy a place at the bottom of a sequence that starts with clusters of galaxies and continues through galaxies and stars. All of these structures assemble and develop under the pull of gravity—their own weight keeps them together. All except the planets have similar compositions that are dominated by hydrogen and helium. Thus planets, in breaking with this uniformity, are more than just the products of gravity: they present the richness of form that the full table of elements—chemistry—can afford.

Imagine a planet that is larger and more massive than Earth but smaller than Uranus. Would a planet like this have deep water oceans—being a true water world—or would it be a dry planet with huge volcanoes billowing smoke high into a thin atmosphere? This is what we are about to explore. First, however, we have to find them.

I
n 1543 Nicolaus Copernicus set in motion events that transformed science and, through technology, human society. His insight—simplifying the architecture of the cosmos and placing the Sun, not Earth, in the center of the planetary system—was essential to the scientists of the next two generations (particularly Galileo and Newton) and the creation of modern physics. The Copernican revolution went directly to the heart of the question about humankind's place in the world. Many thinkers, most famously Dutch physicist Christiaan Huygens (1629–1695), jumped from the Copernican view of Earth as just another planet to the possibility of life on other planets.
¹
In 1686 Bernard de Fontenelle popularized the possibility of extraterrestrial life in
Conversations on the Plurality of Worlds,
and it reached a culmination 300 years
later in books and movies like
War of the Worlds
and
Star Trek
.

Ironically, these conjectured other planets did not materialize for 450 years. Even the nearest stars turned out to be very, very far away; discerning the tiny planets that may orbit them required four centuries of technological development. Now we are finally within reach of completing the Copernican revolution by discovering analogs of the Earth and the Solar System.

What makes extrasolar planets difficult to find is their distance and the fact that they are orbiting stars that are far bigger and brighter than they are. Typically a star is 1 billion to 10 billion times brighter than any orbiting planet, at least in visible light. This is a huge contrast ratio. To make things worse, since the observer is far away, star and planet appear very close to each other in the telescope. Taken separately, the high contrast ratio and the apparent closeness of star and planet are solvable. Together, they have been nearly intractable.

The telescopes that have been in operation during the past twenty years, including the Hubble space telescope, are capable of collecting light from objects fainter than 10 billion times the brightness of the nearby stars. This is done in the same way that a photographer takes a picture at dusk—by keeping the camera's shutter open longer. Taking a longer exposure allows more light to accumulate on the detector inside the camera, revealing very faint objects. The famous image of the Hubble Deep Field was obtained by taking a thirty-three-hour exposure in visible light, revealing thousands of distant galaxies.
²

Many of the extrasolar planets known today could be detected by a very long exposure like that, except that the star makes a huge bright smudge in the middle of the image. The star will be “overexposed,” as a photographer would say, and its light would be scattered all over the image. In fact, it could even damage the detector. Somewhere, lost in this scattered stellar light, is the faint light speck of the planet. That is why discovering a planet orbiting a normal star is such a big challenge.

Solutions have been proposed.
³
One method is to try observing the star and planet in other types of light. The star-to-planet ratio might be a billion to 1 or 10 billion to 1 in
visible
light. But as we know, light is a mixture of colors—waves of different length (or wavelength). These waves, when spread out according to wavelength (as done by a prism, for example) comprise a spectrum, as when water droplets turn sunlight into a rainbow. Consequently, applying a prism and looking for wavelengths in which the ratio between star and planet isn't so great might help.

This does work in some cases. For very hot planets, such as 51 Peg b, the star-to-planet contrast ratio improves a thousandfold (down to 10
⁷
) when observed in infrared light. Infrared is light of longer and longer wavelength, beyond what our eyes see as red light; our skin detects it as heat. A hot planet stands out better in infrared light next to its star because it “shines” with its own heat. A hot Jupiter can have a temperature of 1,500 to 2,000 K, which is much hotter than Earth (at 287 K) but is comparable to the Sun (at 5,800 K). Nevertheless, the 10
⁷
contrast ratio is still daunting. Recently,
infrared observations of known extrasolar planets have succeeded in special cases, but they still don't yield images, and the method is still not used for discovery.
⁴

What other star-to-planet comparisons could we exploit? First, there is mass and then size; for both of these the star-to-planet ratios are much more favorable. For example, the Sun is 1,050 times more massive than Jupiter—so their star-to-planet mass ratio is 10
³
. That is much more manageable than 10
⁷
. With sizes, things get even better—the Sun is “just” ten times the size of Jupiter (and just 109 times the size of Earth)! This sounds good in theory, but how can we use it in practice?

Let's look at the star-to-planet mass ratio, since methods that exploit it have been the most successful and popular so far. The mass of an object determines its gravity—a more massive body exerts a stronger force (or pull). Thus the Sun makes Jupiter revolve around it in an eternal bind. But wait! Is Jupiter orbiting around the Sun like an anonymous slave, or are Sun and Jupiter waltzing their way through the Galaxy?

A waltz it is! To every force there is an equal and opposite reaction force, so the Sun and Jupiter balance each other around their “center of mass,” which is a virtual point that is always on the line that joins them. They
both
orbit around the center of mass, just like a dancing couple. The Sun, being a thousandfold more massive, keeps their center of mass very close to itself, yet that virtual center is not inside the Sun. The Sun-Jupiter center of mass is about 7 percent of the solar radius above the surface of the Sun. To a careless observer from a distant star this might seem indistinguishable from Jupiter just revolving around the center of the Sun. But an
astute and observant astronomer would see the waltz (or wobble, if you will) of the Sun as it orbits around the center of mass with Jupiter. The beauty of this trick is that the astronomer could observe the wobble of the Sun even if unable to see Jupiter in any other way! This is an indirect method of discovering a planet.

There are several practical ways to exploit the star-to-planet mass ratio in order to discover extrasolar planets. Three of them make use of detecting the wobble of the parent star and one exploits the mass ratio in a snapshot of sorts. A star's wobble can be detected directly by carefully observing the position of the star with respect to other stars over a period of time longer than the orbital period of the putative planet. This method—astrometry—allows us to isolate influences on a star's behavior caused by orbiting planets, and not by the Universe at larger scales. It has been tried for many years, at least since the early twentieth century, but turned out to be very demanding. Recently, the Jet Propulsion Laboratory at NASA developed the technology needed to achieve astrometry at the required precision from space, so the method might still deliver in the future.
⁵