Exoplanet Exploration
What are exoplanets?
Throughout recorded history and perhaps before, we have wondered about the possible existence of other worlds, like or unlike our own. The earliest understanding of the solar system showed us that there were indeed other worlds in orbit about our Sun, and steadily growing understanding of their natures shows that all are dramatically different from Earth, and mostly very different from one another. As we came to understand that the stars in the sky are other suns, and that the galaxies consist of billions of stars, it appeared a near certainty that other planets must orbit other stars. And yet, it could not be proven, until the early 1990’s. Then, radio and optical astronomers detected small changes in stellar emission which revealed the presence of first a few, and now many, planetary systems around other stars. We call these planets “exoplanets” to distinguish them from our own solar system neighbors.
How we know that there are planets around other stars?
Most of the detected exoplanets have revealed their presence by small effects that they have on their star. As planet follows its orbital path, the star follows a complementary motion of its own. This is a tiny effect proportional to the planet/star mass ratio – in the case of the solar system, the Sun moves in synch with the Earth at the speed of a slow dance – currently too slow to readily detect in a distant system. The motion of the Sun in synch with Jupiter, however, is closer to a fast run – and in favorable cases it can be detected by several methods. The motion of the host star can be measured as a shift in its spectrum (the Doppler shift) or as a change in its position on the sky (astrometry). In both cases these are very challenging measurements and require exquisitely sensitive instruments. Exoplanet orbits presumably have random orientations, and in some cases the orbit carries the planet between us and its star. Then the exoplanet might be detected by the decrease in the light from the star. Such transits have been observed, and a number of planets discovered by this method.
Another effect that can reveal the presence of a planet around another star is the bending of light from background stars by the gravitational field of an intervening star. If the intervening star has an orbiting planet it may alter the gravitational lensing effect in a noticeable way (microlensing). The large majority of the several hundred known extrasolar planets have been discovered by the Doppler technique, and other methods are contributing more significantly as they are refined and the number of detected exoplanets continues to increase steadily.
What do we know about our exoplanet neighbors?
Although the details are not entirely understood, it is known that stars like the Sun form from spinning protostellar disks of gas and dust. The Earth and other planets of the solar system are believed to have developed from the remains of that disk, and there is no reason to believe that the same process would not be effective throughout the galaxy. Thus a first guess might be that other planetary systems would be like the solar system.
Artists concept of a protostellar disk feeding dust and gas into the accreting star in the center, and providing a reservoir of material for the formation of an orbiting system of planets.
However, the first detections of exoplanets revealed bodies which are utterly unlike any solar system planet – and subsequent discoveries have shown that many exoplanet systems are very dissimilar from ours. In some exosystems, planets as massive as Jupiter orbit so close to their star that they are heated to high temperature and their upper atmospheres are swept into space. In other systems, planets follow elongated orbits (in contrast to the nearly circular orbits of the solar system). However, our studies of exoplanets are just beginning, and it is not possible to be sure what will prove to be “typical” planets among our neighboring stars. Will most planet systems prove to be much like our own, or are we exceptional in more ways than we can imagine? Only years of further study will tell.
Some slim evidence is accumulating that exoplanet systems which resemble the solar system will be found. A recent discovery has shown that the star 55 Cancri, 41 light years away, has a system of 5 planets, with distributions somewhat similar to the solar systems inner planets (though with much higher masses). As our measurements become sensitive to lower masses, some astronomers believe that we will find many such systems with a substantial complement of planets (perhaps even dynamically full – that is, containing as many planets as can coexist in orbital harmony).
In other reports, a number of planets with masses near that of Earth have been detected. The results are few, but because the measurements are very difficult, the detections are considered significant and possibly indicative of many more to be found in the future. Again, only years of study will tell.
What do we want to learn about exoplanets?
A thorough understanding of exoplanets will tell us much about how our solar system formed, why it has small, rocky planets near the Sun, why it has gas giant planets far from the Sun, why the Earth has the conditions and chemicals that can support life, and why conditions on other planets are hostile to life. Theories of planet formation and evolution are incomplete, but offer specific predictions. Detections of exoplanets are already testing, validating, and in some cases invalidating, details of these theories.
Perhaps the most interesting question, and one of the most difficult to answer, concerns the uniqueness of the Earth. Are there planets similar to the Earth around other stars? For example, alpha Centauri A, a star much like the Sun, is one of our nearest stellar neighbors, at only 4 light years distance. Does it harbor a planet like Earth? And what is an Earthlike planet? Could it be a little smaller or larger and still qualify? How about hotter or colder? Scientists have considered this question, and while the discussions continue, the prevailing opinion is that a planet between ¾ and 3 times the Earth size, and somewhat colder or warmer than Earth, could possibly maintain the conditions that would enable life. But these questions are far from simple, and can engage consideration of factors as varied as plate tectonics, magnetic fields and the evolution of the Sun. In short, studying the habitability of exoplanets will teach us a great deal about the conditions for habitability of Earth – a subject of considerable interest us and our descendants.
Search for signatures of life
Does life exist on any other planet beyond our own Earth? This fundamental question has been on the minds of many philosophers throughout the human history but only now are we approaching the technological development that may allow us to start seeking experimental answers. For practical reasons, life could be detected by remote sensing only from the byproducts that biological activity leaves in the atmosphere or on the surface of the host planet. Provided we can find an Earthlike planet in the habitable zone around another star, work will begin to characterize it as best as possible to look for signatures of life. These include the presence of significant amounts of water, methane, ozone and other gases suggesting bioactivity in the atmosphere, and possibly large amounts of vegetation on the surface (the red edge). This is a daunting task and will require first separating the light of the planet from the blinding glare of the host stars and then obtaining a diagnostic spectrum of the vanishingly faint planet in several wavelengths.