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.
What is the difference between a planet and a star?
Stars are huge luminous balls of gas powered by nuclear reactions at their centers. The enormously high temperatures and pressures in the core of a star force atoms of hydrogen to fuse together and become helium atoms, releasing tremendous amounts of energy in the process. Planets are much smaller with core temperatures and pressures too low for nuclear fusion to occur. Thus they emit no light of their own. When you see Venus or Jupiter in the night sky, you’re really seeing sunlight reflected by those planets back to you.
Some planets, like Earth and Mars are solid rocky bodies, but others, like Jupiter and Saturn are mostly gas and liquid. Jupiter, the largest planet in our solar system, is roughly 300 times more massive than the Earth, but only one – thousandth the mass of the Sun. However, had Jupiter been 75 times more massive, it would just have been large enough for the pressures and temperatures at its core to ignite nuclear fusion, and the Earth would have had two Suns in our skies.
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.
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