The Life of Super-Earths (2 page)

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I
n October 1995, I was attending a conference in Florence, Italy, that beautiful old city where the Medicis were the patrons of astronomy during the seventeenth century. I was there to exchange new ideas and thinking with my colleagues. Then during an unguarded moment of casual conversation, as often happens, a bold new concept exploded amid my deeply held presumptions.

At the day's end, a couple of us were talking to Swiss astronomer Michel Mayor about his discovery of a small companion—a planet about the size of Jupiter—around a star named 51 Pegasi. The claim itself was not a “wow” moment; such claims had come and gone in decades past. What really caught my attention was that Michel and his graduate student, Didier Queloz, had measured the orbital period in
days, not in years, as one would expect. This new planet circled its sun in just four hundred days!

I was incredulous.

Okay, stars are my specialty, not planets, but I know the basics, and this did not fit. As far back as my last year of high school I had known about the Kant-Laplace model of the formation of our Solar System. Although you may know Immanuel Kant as a philosopher, as a young man he was an astronomer and an Isaac Newton groupie. He was at the University of Koenigsberg, today's Kaliningrad on the Baltic Sea, and he used Newton's new calculus and theoretical mechanics to solve an obvious but unexplained feature of the Solar System.

Astronomers before Kant had noted that all planets orbit the Sun in the same plane and in the same direction, which is also the direction in which the Sun spins. Most planets spin that way as well. Kant offered an elegant solution for this by analogy with Saturn's rings. Planets form from particles circling the sun in a rotating flat disk, and the conservation of angular momentum explains its flattened shape.
^a
(Because his publisher went bankrupt, Kant didn't get the credit due him at the time, as recounted in
The Discovery of Our Galaxy
by my old mentor, Charles Whitney.)
¹
Pierre-Simon Laplace added mathematical rigor to Kant's ideas in 1796, and the Kant-Laplace
model has survived 250 years of critiques, changes, and improvements while retaining its basic foundations.

 

FIGURE1.1
.
The newly formed star is surrounded by an orbiting disk of gas and dust, the material from which planets form. The disk is heated by the star and there is a curve at a distance where its temperature drops below freezing, known as the snow line. It is outside this line that snowflakes add to the dust in the formation of planets and help create gas giants like Jupiter.

There was something else that made me find Michel's discovery a bit hard to believe. According to the modern version of the Kant-Laplace model, there is a curve, roughly two to three times the distance of the Earth from the Sun, at which the temperature of the gaseous disk surrounding a star falls to just 170 Kelvin, or 150 degrees below zero Fahrenheit, at which point water and ammonia molecules in that rarefied atmosphere form ice grains and snowflakes.
²
These two light materials, as well as, ultimately, hydrogen, combine with dust particles and grow into giant gas planets orbiting the sun. Within the so-called snow line, dust particles, with no ice grains and snowflakes to aid their growth, combine to form small, dense planets (see
Figure 1.1
on the preceding page). This is the beautifully simple explanation for the makeup of our solar system, gas giant planets orbiting the sun farther out and taking years to complete their journeys, and small, rocky planets orbiting closer in. So you can see why I was surprised by Michel's claim—there was no way a Jupiter-like gas giant planet could have ever formed inside the snow line. And orbiting 51 Pegasi, a star like our Sun, in just four hundred days—that just seemed impossible.

At the press conference the next morning, I found out I had been mistaken about the four hundred days.

It was four days!

Somehow my brain had locked onto the incredible figure and multiplied it by a factor of one hundred. Yet there was
Michel, with the evidence to back his claims, showing that the orbital period of the new planet was 4.2 days!

My deeply held preconceptions fell apart like ice grains and snowflakes meeting the Sun. It was a powerful—and humbling—lesson.

News of many more planets has followed the discovery of 51 Peg b.
³
Geoffrey Marcy and Paul Butler in California, already pursuing a similar project and technique, discovered several interesting planetary systems within months of Michel's announcement, allaying any lingering doubts that what Michel interpreted as planet 51 Peg b might have been an unusual property of its star. It was also easier to go back to an early find and accept it as a possible planet—the companion of star HD 114762, discovered in 1989 by my colleague and pioneer planet hunter, David Latham, and his collaborators.
⁴
It was also possible to see why the pioneers of the technique, led by Gordon Walker of the University of Victoria in Canada, had failed to discover a single extrasolar planet: they had done a systematic search from 1986 to 1995 but looked for planets with periods of ten years or longer, which limited the number of stars they could monitor. With some bad luck, the search ended empty-handed.
⁵

Planets orbiting other stars, dubbed extrasolar planets or exoplanets, now number in the hundreds—about 600 at the time of this writing. All of them lie in our Milky Way Galaxy, relatively close to home, most within a circle of 500 light-years, although a handful are as far away as 5,000 light-years. More than sixty of these planets are similar to 51 Peg b
and are referred to as “hot Jupiters” (
Figure 1.2
). This number, which is fairly high, reflects the fact that the planets are easy to find, not that they are numerous. These planets, which at first seemed so anomalous (how could they have formed so close to the heat of their stars?), ended up having an explanation that didn't require throwing out the Kant-Laplace model. The hot Jupiters opened our eyes to the phenomenon of planet migration, the result of slow changes to a newly formed planet's orbit due to its interaction with the disk of gas and dust. As the orbiting planet raises density waves in the disk, its orbit can spiral inward or outward. In most cases, the shift is inward; the result is hot Jupiters.
⁶

 

FIGURE1.2
.
The orbit of the first “hot Jupiter” planet, 51 Peg b. The two orbits are shown at the same scale. The distance from the Earth to the Sun is 93 million miles, from 51 Peg b to 51 Peg, 5 million miles.

So while my experience in the beautiful old city of the Medicis took some time to sink in, when it did, I was deeply inspired to find answers to the questions that just days before I had taken for granted.

Thirteen years later Michel and I met again at the same conference. This time Michel described a bounty of small planets, perhaps like Earth, that he had discovered. I reported, based on computer calculations, on the strange worlds some of them might be. These smaller planets are more numerous and diverse than anyone had expected—searing hot planets with iron rain, atmospheres with 1,000 mile an hour winds, planetary systems with two suns, a planet that literally skims the surface of its star once every three months, and more.

Today we stand on the threshold to new worlds—planets that we could call home, planets that someone else might call home already. The search for them has spawned a new space race: the race to discover an Earth twin planet. The
zeal and effort going into this race may seem odd and unjustified. Even for scientists there is no overwhelming benefit in discovering an Earth twin, because to study the properties of Earth-like planets they could rely on bigger ones, which are much easier to find. Yet everyone agrees that this is a historic moment. What gives rise to the extraordinary excitement of this race is the human yearning for meaning and belonging. It is the twenty-first-century version of the age-old question of “the Other,” but on a grand scale.

The question of the Other is about how a conscious human being perceives his own identity: Who am I and how do I relate to others? It arises front and center during first encounters. Human history is full of first encounters:
Homo sapiens
encounters
Homo neanderthalensis
somewhere in today's Europe, Mayans encounter Spanish conquistadors in Central America, and so on.
⁷
The time of first encounters on our planet is now over. For better or worse, we humans—all of us—know about each other. The present generation of
Homo sapiens
has a global awareness, a sense of social connectedness, and an understanding of a common genetic makeup. The end of the twentieth century marked a real watershed in this sense.

The discovery of new worlds orbiting distant stars offers a fresh opportunity to contemplate a first encounter. As in the past, humans approach it with both insatiable curiosity and fear, with mixed, very strong emotions. As in the past—amazingly, despite all our modern technology and the visions of
Star Trek
—the new worlds we have just begun to uncover
are enshrouded in mystery and full of surprises. And we will never stop exploring, as T. S. Eliot famously wrote: “We must never cease from exploration. And the end of all our exploring will be to arrive where we began and to know the place for the first time.”

I
n the mid-1990s the world of planets was a small one that comprised the nine planets of the Solar System; Pluto's “planethood” had not yet been challenged. Still, those planets represented a diversity of environments not imagined. The cameras aboard the flotilla of spaceships exploring our Solar System had shown us those exotic places and taught us the basics of comparative planetology. We couldn't be sure just how important planets, let alone life, were to the Universe at large. Today, for the first time, scientists can look at both planets and life as integral parts of the Universe and its history. In what follows we'll do just that.

 

The planets in our Solar System form two groups—the gas giants and the terrestrial planets. Jupiter, by name and by physical stature, is the ruler of them all. How the ancients
could have sensed that is a mystery, since it took scientists more than two and a half millennia to measure Jupiter's mass and size and confirm its enormity for a fact.
¹
Four hundred years ago in Padua, Galileo Galilei first used his unusual optical device, the telescope, to look at Jupiter. Galileo saw a planet, not a point-like star, orbited by four moons. He called them stars—Medicean stars, naming them for his Florentine patrons, the Medicis—the distinction between stars and planets not having been clarified yet. Now we call them the Galilean moons of Jupiter; their orbits help us discern the planet's gravitational pull and measure its mass.
^b
That measurement was one of the triumphs of Isaac Newton's law of gravity in the generation of scientists that followed Galileo, and it inspired young thinkers like Immanuel Kant to figure out how the planets formed. It showed that Jupiter had the mass of more than 300 Earths and more than two times the mass of all the other planets taken together. Jupiter is a giant indeed, rivaled only by its distant second—the ringed planet Saturn.

Jupiter is a gas giant planet—we know that from its average, or mean, density. Its radius is ten times larger than
Earth's, which makes Jupiter's volume a thousandfold larger. Given that Jupiter has only 300 times more mass, it must be made of stuff that is less dense than our rocky Earth. Indeed, Jupiter and Saturn are composed mostly of hydrogen and helium, the two most common and lightest elements in the Universe, very similar to the makeup of the Sun and the stars (
Figure 2.1
).