Planets Around Other Stars
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.
The distribution of Kepler planet candidates by size as of January 2015.
Image Credit: NASA Ames/W Stenzel
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.
Evidence is accumulating that exoplanet systems which resemble the solar system are being found. 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 and does life exist on any other planet beyond our own Earth?