Zooming in on the universe has not been easy or quick. Humans watched the night sky for tens of thousands of years so that by the time early civilizations arose in areas like Mesopotamia, China, and Pre-Colombian America, amazingly detailed knowledge had been gained of the movements of the planets and other heavenly objects visible to the naked eye.
It was not until 1610 that a newfangled gadget called the telescope allowed Galileo to go beyond the limits of the eye and make his famous observations of four moons orbiting Jupiter.
Many discoveries were also made of the universe beyond our solar system but it still was not until 1923 that telescopic instruments advanced to a sufficiently sensitive level to allow Edwin Hubble to discover that many of those stellar lights were not stars but galaxies, i.e. conglomerations of billions of stars just like our own Milky Way.
And it wasn’t until the mid-1990s that a star was resolved by a telescope as a disc rather than a point blur of light. The star was the red super-giant Betelgeuse, whose diameter is about as big as the orbit of Jupiter. Only a handful of similar giants have been subsequently resolved .
So making the next step and zooming in on planets around other stars is clearly a terrifically difficult task. I think it is fair to say that most people, including most astronomers, assumed up until the 1980s that such discoveries would require gigantic telescopes not available until far into the future; at least for spotting planets significantly smaller than Jupiter.
In Five Billion Years of Solitude: The Search for Life Among the Stars, Lee Billings tells the story of how astronomers successfully developed methods to detect planets around other stars without seeing them directly. The trick is to detect the effects of an exoplanet on its host star.
The first method to find an exoplanet successfully involves measuring shifts in the frequency of a star’s light as a planet orbits the star. The orbiting planet causes the star to wobble and this wobble results in an increase in the frequency of light when the star moves towards us and a decrease when the star moves away from us. This is a very tiny effect but by monitoring hundreds of spectral lines for the telltale signs of Doppler shifts, the combined statistics of the shifts in all those lines creates a significant signal in the data as more and more orbits of the planet are observed. Groups in Canada, Switzerland and the US found the first exoplanets using this method in the early 1990s.
(In the book, Billings touches on the professional rivalries and squabbles that have arisen in the highly competitive exoplanet-finding field. Here is an article by him about one such battle: The Ugly Battle Over Who Really Discovered the First Earth-Like Planet – WIRED.)
This Doppler shift method (officially referred to as the radial-velocity method) is biased towards big exoplanets orbiting close to their stars. While our sun has little Mercury as its closest inner planet, it turns out that there are many stars out there with massive exoplanets orbiting much closer to their stars than Mercury is to the Sun. Over time this method has gotten increasingly refined and smaller planets, farther out from their star have been observed.
Another method is to look for the slight dimming of a star when one of its planets passes between us and the star. This transit method requires that the plane of the planet’s orbit is oriented edge on from out point of view. This would seem to be very rare but there are so many stars out there that it happens often enough to give us plenty of cases to observe.
The Kepler space observatory has been spectacularly successful using the transit method. The Kepler team has accumulated a list of about 1000 confirmed exoplanets and over 4000 candidates are still under study. Most of the Kepler exoplanets are also large and too close to the stars to allow for life as we know it. However, a small subset of rocky planets similar in scale to the earth have been detected in “habitable zones” in which their orbits receive sufficient energy from their suns to allow for liquid water if the planets have dense enough atmospheres. That a handful of such candidate rocky exo-earth candidates have been detected is an enormous accomplishment.
The book, however, is not a technical guide to exoplanet detection. As the full name of the book implies, Billings puts the exoplanet discoveries in the broader context of the search for life beyond earth and for extraterrestrial intelligence (SETI). He focuses particularly on Frank Drake, the leading pioneer in SETI, who published his famous Drake Equation in the 1960s for estimating the number of advanced civilizations in the galaxy. Or guesstimating is more accurate. Most of the parameters in the equation, such as the probability that a star has planets and the fraction of such planets that could support life, were poorly known or not known at all. The exoplanet discoveries finally provide hard data to determine some of these parameters.
Billings also reviews the formation of the solar system and the geologic and biologic history of the earth. Earth’s development provides provide clues as to what to look for when seeking exoplanets with life.
Zooming in on earth-like planets and seeing them directly remains a key goal for exoplanet searchers. The most straight-forward way to do this is to build observatories in space and use special techniques to mask out the tremendous glare of the star so that the meager reflected light from the exoplanet can be examined. Most of the designs for such observatories will take considerable resources and Billings laments the lack of funding for such projects. He fears that just as we are finally gaining the capability to see and study distant earths, the implementation of that capability is receding into the future as NASA’s budget remains flat or falls.
(Recently Billings wrote about designs for relatively low cost space observatories specialized to see earth-sized planets around nearby stars if such planets exist : Planet Hunters Bet Big on a Small Telescope to See Alien Earths – Scientific American.)
It has been nearly five billion years since the formation of our solar system and our earth. Billings message is that we have now proven that earth is not the only rocky planet in the Milky Way. The next step is to study these exo-earths and determine if humans remain in solitude or have companions in our galaxy as well.