For most of human history, the only known planets beyond Earth consisted of those visible to the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. The development of telescopes enabled the detection of two fainter, more distant planets — Uranus (1781) and Neptune (1846).
Since those discoveries, astronomers have speculated about the possible presence of planets in the outer reaches of our solar system far beyond Neptune's orbit. These hypothetical planets have been referred to by different names, such as Nemesis, Planet X and (most recently) Planet Nine. Astronomers have not found any direct evidence for additional planets in our solar system, but they continue to search for them with each new generation of more powerful telescopes.
Do planets exist around other stars? If so, they would be a million times fainter than the stars that they orbit, so they would be nearly impossible to see in the bright glare of their stars. However, astronomers have developed methods of indirectly detecting the presence of unseen planets around other stars. For instance, when a planet orbits a star, the star also experiences an orbit of its own, which is so small that it appears as tiny wobble. If the star is a pulsar (the remnant left behind when a star much more massive than the Sun undergoes a supernova explosion), its wobble can be detected through careful measurements of the arrival times of its pulses of light. In 1992, Penn State Professor Alex Wolszczan used this method to discover the first known planets outside the solar system, which orbit a pulsar at a distance of 2,300 light-years from Earth.
To search for planets around normal stars like the sun, astronomers have primarily used the Doppler and transit methods. The Doppler method involves the detection of a star's wobble via the redshift and blueshift of its light as it repeatedly moves toward and away from Earth. With the transit method, a planet can be detected if its orbit is aligned so that it passes directly between its star and Earth, leading to a brief dimming of the star's light.
The first known planet around a star like the sun was discovered with Doppler measurements in 1995, which consisted of a gas giant similar to Jupiter in an orbit much smaller than that of Mercury, which has been dubbed a "hot Jupiter." Since then, a few thousand planets have been found using the Doppler and transit methods, some of which appear rocky like Earth.
A few of these rocky planets may have the appropriate temperature for liquid water, which is an essential ingredient for life as we know it. NASA's Transiting Exoplanet Survey Satellite, which was launched in April, is expected to discover several hundred rocky planets in the next few years. For the rocky planets most similar to Earth, NASA's James Webb Space Telescope will be able to study their atmospheres for signatures of life. Closer to home, scientists hope to search for evidence of life elsewhere in our solar system with new missions to Mars and water-bearing moons in the outer solar system like Europa. With new space probes and telescopes of this kind, our knowledge of planets in the universe should continue to grow rapidly for many years to come.
OLLI at Penn State — open to adults who love to learn — offers 80 courses this summer semester. Kevin Luhman will lead a course on Planets Near and Far in July. To receive a free catalog for the summer semester, call OLLI at Penn State at 867-4278 or visit olli.psu.edu.