For centuries astronomers speculated about whether stars other than our sun had planetary systems. The answer to that question came in 1995 with the first confirmed discovery of an exoplanet orbiting a sunlike star approximately 50 light-years away in the constellation Pegasus.
As of Oct. 1 the Kepler space telescope and other observatories have discovered 3,851 confirmed planets in 2,871 systems, with 636 systems having more than one planet. The nearest is a mere 4.2 light-years distant.
In August, NASA’s Transiting Exoplanet Survey Satellite, or TESS, began surveying the galaxy. Already it has detected two new exoplanet candidates. More are sure to follow.
Because the light from a star is a billion times brighter than the light reflected from a planet, it is difficult to observe exoplanets directly. Astronomers use a variety of methods to search for planets. Two of these are easy to understand and have yielded excellent results.
One measures the wobble of a star; the other measures diminished light from an eclipse of the star.
A planet and a star constitute a gravitational system where they orbit each other around their combined center of mass. Because the star is much more massive than a planet, its motion is smaller by the ratio of their masses.
This causes the star to wobble as the planet orbits. The wobble is both back and forth and side to side as seen from our perspective.
One type of observation uses the Doppler shift to measure the back-and-forth motion of the star. This shows up in the spectrum of the star as a slight blue shift as it wobbles toward us and a red shift as it wobbles away.
It requires an incredibly sensitive instrument to measure these slight changes, but spectral instruments routinely measure such small shifts as we measure distant galaxies that are red-shifted.
For example, the sun moves by about 40 feet per second due to Jupiter but only about 3-1/2 inches per second due to Earth. However, modern spectrometers can detect velocity variations down to 3.3 feet per second.
Another method measures the side-to-side wobble directly. Again, this is extremely small but well within the capability of modern astronomical instruments.
The photometric method measures the small amount of change in visual brightness of the star as the planet passes in front of it. This works only if the planetary orbit is in the plane of our perspective.
The amount of dimming depends on the size of the star and the planet. Most dimming signals are considerably smaller than 1 percent. For example, an Earth- size planet transiting a sunlike star would produce a dimming of only 80 parts per million (0.008 percent).
Since its launch in 2009, the Kepler space telescope has continually monitored about 150,000 stars and kept track of their brightness over time, allowing for discovery of several thousand exoplanets. To date only a handful of these are Earth-like, and even fewer orbit in the habitable zone of stars that provide the conditions for life.
Projections from Kepler data indicate that there could be as many as 40 billion rocky, Earth-size exoplanets orbiting in the habitable zones of sunlike stars and red dwarfs within our Milky Way galaxy. The nearest planet estimated to be orbiting a sunlike star may be only 12 light-years away.
Although we cannot measure “life” directly, astronomers have confidence that soon we will be able to reliably detect the chemicals of life such as oxygen, methane and nitrogen in the atmospheres of distant planets.
Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii. edu.
