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MOST STARS ARE ORBITER BY PLANETS

Hot and super Jupiters

The exoplanets discovered so far have added a host of weird worlds to the neat family portrait that is the sun’s planetary system. For example, 51 Pegasi b was the first of many “hot Jupiters.” These have a mass similar to Jupiter’s and a large size that shows that they are mostly made of gas. 51 Pegasi b is half as massive as Jupiter, but is slightly larger. This gas giant orbits its sunlike star every four days. That means it is much closer to its star than Mercury is to the sun. Such proximity means it is tidally locked to the star— one side always faces the scorching stellar surface, and the other always faces away. Many hot Jupiters have been found. They have confounded scientists, who are trying to understand how gas planets can exist so close to a star without evaporating. Some exoplanets are dozens of times more massive than Jupiter, and are known as “super-Jupiters.” These super-Jupiter planets do not appear to grow in size as their mass increases. For instance, Corot-3b is a super-Jupiter that is 22 times as heavy as Jupiter but more or less the same size, due to its gravity holding its gaseous contents together. Astronomers have calculated that the density of Corot-3b is greater than that of gold and even osmium, the densest element on Earth.

Kepler 10b in the Draco constellation is shown transiting its star in an artist’s impression. Its extremely hot surface temperature and dizzying orbit mean life there is improbable.

Brown dwarfs and rogues

When a super-Jupiter reaches 60 Jupiter masses, it is no longer regarded as a planet, but as a brown dwarf. A brown dwarf is essentially a failed star—a ball of gas that is too small to burn brightly through nuclear fusion. The brown dwarf and its star are seen as a binary star system, not a planetary one. Some super-Jupiters and small brown dwarfs have broken free of their star to become free-floating rogue planets. One, named MOA-2011-BLG-262, is thought to have a satellite, and could be the first exoplanet found with an exomoon.

Another class of planet are called the super-Earths. These have a mass 10 times that of Earth but less than that of an ice giant like Neptune. SuperEarths are not rocky but made from gas and ice: alternative names for them are mini-Neptunes or gas dwarfs.

The size of the habitable zone (green) depends on the size of the star. The red zone is too hot, while the blue zone is too cold. The habitable zone is closer to cooler stars than it is to hotter stars. The size of a planet, the shape of its orbit, and the speed of its rotation between night and day also affect its habitability.

Living planets

Earth’s solar system has terrestrial planets (planets with a rocky surface), of which Earth is the largest. So far, exoplanet searches have struggled to find many terrestrial planets, because they are generally small and beyond the sensitivity of the planet detectors. The first confirmed terrestrial exoplanet was Kepler-10b, which is three times the mass of Earth and is so close to its star that it orbits once an Earth day and has a surface temperature that would melt iron. Life seems highly unlikely there, but the hunt continues for rocky planets that might be more hospitable.

Astrobiologists—scientists who search for alien life—focus on the particular conditions that all life needs. When choosing likely places to look, they assume that alien life-forms will require liquid water and carbon-based chemicals, just like life on Earth. Living planets would also need an atmosphere to shield the surface from damaging cosmic rays and to act as a blanket that retains some of the planet’s heat during the night.

The region around a star where the temperatures would allow planets to have liquid water, carbon chemistry, and an atmosphere, is known as its habitable zone, also called the “Goldilocks zone”—like Baby Bear’s porridge in the fairy tale, “not too hot, not too cold.” The size and locations of habitable zones depend on the activity of the host star. For example, if Earth were orbiting a K-type star, an orange dwarf that is considerably cooler than the sun (the sun is a G-type, or yellow dwarf), it would need to orbit at about one-third its current distance to receive the same amount of warmth.

Of the thousands of exoplanets that have been identified, only a tiny proportion are candidates orbiting in their star’s habitable zones, with Earthlike conditions for life—rocky surface with liquid water. Typically, they are larger than Earth, and very few have good prospects for being Earth-like. If and when Earth-like planets are found, astrobiologists will look at the atmospheric chemistry for signs of life, such as the presence of elevated levels of oxygen, produced by photosynthesizing life-forms. How life evolved from nonliving material on Earth is still a mystery but the study of Earth-like planets may throw light on that process. Even if life is found, it is likely that most extraterrestrial natural histories will not have moved beyond microorganisms. As every step toward evolving more complex life-forms becomes ever more unlikely, so alien civilizations that match humankind’s will be a lot less common. However, if only G-type stars, like the sun, are counted, there are about 50 billion in the galaxy. It is estimated that 22 percent of them have an Earth-like planet in their habitable zones, which equals 11 billion possible Earths. Adding in other types of stars such as orange and red dwarfs, that number rises to 40 billion. Even if the probability of civilizations evolving is one in a billion, the chances are that humankind is not alone.

“If we keep working as well and we keep being as enthusiastic … the issue about life on other planets will be solved.” Didier Queloz

MICHEL MAYOR

Michel Mayor was born in Lausanne, Switzerland, and has spent most of his career working at the University of Geneva. His interest in exoplanets arose from his earlier study of the proper motion of stars in the Milky Way. To measure this motion more accurately, he developed a series of spectrographs, which eventually culminated in ELODIE. The ELODIE project with Didier Queloz was initially intended to search for brown dwarfs—objects that were bigger than planets but not quite large enough to be stars. However, the system was sensitive enough to spot giant planets as well, and, following their 1995 discovery, Mayor is currently the chief investigator at the HARPS program for the European Southern Observatory in Chile. His team has found about half of all the exoplanets discovered to date. In 2004, Mayor was awarded the Albert Einstein medal.

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