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 allowed the detection of two farther and farther planets: Uranus (1781) and Neptune (1846).
Since those discoveries, astronomers have speculated about the possible presence of planets in the confines of our solar system beyond the orbit of Neptune. These hypothetical planets have been referred to by different names, such as Nemesis, Planet X and (more recently) Planet Nine. Astronomers have not found any direct evidence of additional planets in our solar system, but they continue to search for them with each new generation of more powerful telescopes.
Are there planets around other stars? If so, they would be a million times weaker than the stars they orbit, so they would be almost impossible to see in the bright glow of their stars. However, astronomers have developed methods to indirectly detect the presence of invisible planets around other stars. For example, when a planet orbits a star, the star also experiences its own orbit, which is so small that it appears as a small oscillation. If the star is a pulsar (the remnant that remains when a star much more mbadive than the Sun experiences a supernova explosion), its oscillation can be detected by 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 planets known outside the solar system, which orbit a pulsar at a distance of 2,300 light-years from Earth.
To look for planets around normal stars such as the sun, astronomers have mainly used Doppler and transit methods. The Doppler method involves detecting the oscillation of a star through redshift and the blue shift of its light as it moves to and from Earth. With the transit method, a planet can be detected if its orbit is aligned to pbad directly between its star and Earth, leading to a brief darkening of the star's light.
The first known planet around a star like the sun discovered with Doppler measurements in 1995, which consisted of a gas giant similar to Jupiter in a much smaller orbit than Mercury, which has been called a "hot Jupiter" " Since then, thousands of planets have been found using Doppler and transit methods, some of which are rocky like Earth.
Some of these rocky planets may have the right temperature for liquid water, which is an essential ingredient for life based on what we know. The NASA Transite Exoplanet Survey Satellite, which was launched in April, is expected to uncover several hundred rocky planets in the coming years. For rocky planets more similar to Earth, NASA's James Webb Space Telescope will be able to study its atmospheres in search of signatures of life. Closer to home, scientists hope to look for evidence of life in other parts of our solar system with new missions to Mars and water moons in the outer solar system such as Europe. With new space probes and telescopes of this type, our knowledge of the planets of the universe should continue to grow rapidly for many more years.
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.
Kevin Luhman is a professor in the department of astronomy and astrophysics and in the Center for Exoplanets and Living Worlds. in Penn State.