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The Development of Exoplanet Detection Methods, Techniques and Ideas Before the Kepler Space Telescope

The Ideas Become Focused

In a previous article of mine, we discussed false positives of exoplanets. So why mention so many mistakes about the search for exoplanets? Let me paraphrase something the Mythbusters are fond of saying: failure is not only an option, it can be a learning tool. Yes, those scientists of the past were mistaken in their findings but the ideas behind them were powerful. They looked at orbital shifts trying to see the gravitational pull of the planets, something that many current exoplanet telescopes do. Ironically enough, the masses as well as the distances from the central stars were also accurate to what is considered the main type of exoplanets: hot Jupiters. The signs were pointing in the right direction, but not the techniques.

By 1981, many scientists felt that within 10 years solid evidence of exoplanets would be found, a very prophetic stance as the first confirmed planet was found in 1992. The main type of planet they felt would be found would be gas giants like Saturn and Jupiter, with a few rocky planets like Earth also. Again, very good insight into the situation as it would eventually play out with the aforementioned hot Jupiters. Scientists at the time began to construct instruments that would aid them in their hunt for these systems, which could shed light on how our solar system formed (Finley 90).

The big reason why the 1980’s was more prone to take the search for exoplanets serious was the advancement of electronics. It was made clear that optics needed a boost if any headway was to be made. After all, look at how many mistakes scientists of the past had made as they tried to measure microseconds of change. Humans are fallible, especially their eyesight. So with the improvements in technology it was possible to not rely just on reflected light from a telescope but some more insightful means.

An example of a barycenter, highlighted as the 'x' in the center.

An example of a barycenter, highlighted as the 'x' in the center.

Many of the methods involve making use of the barycenter of a system, which is where the center of mass is for orbiting bodies. Most barycenters are within the central object, like the Sun, so we have a hard time seeing it orbit about it. Pluto’s barycenter happens to be outside of the dwarf planet because it has a companion object, which is comparable in mass to it. As objects orbit about the barycenter, they seem to wobble when one looks at them edge-on due to the radial velocity along the radius from the orbital center. For far away objects, this wobble would be difficult at best to see. How hard? If a star had a Jupiter or Saturn-like planet orbiting it, someone viewing that system from 30 light-years would see a wobble whose net motion would be 0.0005 seconds of arc. For the 80s this was 5-10 times smaller than current instruments could measure, much less photographic plates of antiquity. They required a long exposure, which would remove the precision needed to spot an accurate wobble (Ibid).

Multichannel Astrometric Photometer, or MAP

Enter Dr. George Gatewood of the Allegheny Observatory. During the summer of 1981 he came up with the idea and technology of a Multichannel Astrometric Photometer, or MAP. This instrument, initially attached to the Observatory’s 30-inch refractor, made use of photoelectric detectors in a new way. 12-inch fiber optic cables had one end placed as a bundle at a telescope’s focal point and the other end feeding the light to a photometer. Along with a Ronch grating of about 4 lines per millimeter placed parallel to the focal plane, allows light to be both blocked and enter the detector. But why would we want to limit the light? Isn’t that the valuable intel we desire? (Finley 90, 93)

As it turns out, the Ronch grating doesn’t prevent the entire star from being obscured and it can move back and forth. This allows different portions of the light from the star to enter the detector separately. This is why it is a multichannel detector, because it takes input of an object from several close positions and layers them. In fact, the device can be used to find the distance between two stars because of that grating. Scientists would just need to examine the phase difference of the light due to the movement of the grating (Finley 90).

The MAP technique has several advantages over the traditional photographic plates. First, it receives the light as an electronic signal, allowing for higher precision. And brightness, which could wreck a plate if overexposed, doesn’t affect the signal MAP records. Computers could resolve the data to within 0.001 arc seconds, but if MAP were to get into space then it could achieve a precision of one-millionth of an arc second. Even better, scientists can average the results for an even better sense of an accurate result. At the time of the Finley article, Gatewood felt it would be 12 years before any Jupiter system would be found, basing his claim on the orbital period of the gas giant (Finley 93, 95).

Using Spectroscopy

Of course, a few unsaid topics arose during all the development of MAP. One was the use of the radius velocity to measure spectroscopic shifts in the light spectrum. Like the Doppler effect of sound, light too can be compressed and stretched as an object moves towards and away from you. If it is coming towards you then the light spectrum will be shifted blue but if the object is receding then a shift to the red will occur. The first mention of using this technique for planet hunting was in 1952 by Otto Struve. By the 1980s, scientists were able to measure radial velocities to within 1 kilometer per second but some were even measured to within 50 meters per second! (Finley 95, Struve)

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That being said, Jupiter and Saturn have radial velocities between 10-13 meters per second. Scientists knew that new tech would need to be developed if such subtle shifts were to be seen. At the time, prisms were the best choice to break up the spectrum, which was then recorded onto film for later study. However, atmospheric smearing and instrument instability would frequently plague results. What could help prevent this? Fiber optics once again to the rescue. Advances in the 80s made them larger as well as more efficient at both collecting light, focusing it, and transmitting it along the entire length of the cable. And the best part is you don’t need to go into space because the cables can refine the signal so that the shift can be discerned, especially when used in combination with a MAP (Finley 95).

Transit Photometry

Interestingly, the other untouched topic was the use of the electronics to measure the signal of the star. More specifically, how much light we see from the star as a planet transits across the face of it. A noticeable dip would occur in the brightness and if periodic it could indicate a possible planet. Mr. Struve was once again an early proponent of this method in 1952. In 1984 William Borucki, the man behind the Kepler Space Telescope, held a conference in the hopes of getting ideas started as to how best accomplish this. The best method considered at the time was a silicon diode detector, which would take a photon that hit it and convert it into an electrical signal. Now with a digital value for the star, it would be easy to see if less light was coming in. The downside to these detectors was that each could be used for just a single star. You would need many to accomplish even a small survey a sky, so the idea while promising was deemed infeasible at the time. Eventually, CCD’s would save the day (Folger, Struve).

A Promising Start

Scientist sure did try many different techniques to find planets. Yes, many of them were misguided but the effort had to be extended as advances were made. And they did prove to be worthwhile. Scientists used many of these ideas in the eventual methods that are currently used to hunt for planets beyond our solar system. Sometimes it just takes a little bit of a step in any direction.

Works Cited

Finley, David. “The Search for Extrasolar Planets.” Astronomy Dec. 1981: 90, 93, 95. Print.

Folger, Tim. "The Planet Boom." Discover, May 2011: 30-39. Print.

Struve, Otto. “Proposal for a Project of High-Precision Stellar Radial Velocity Work.” The Observatory Oct. 1952: 199-200. Print.

© 2015 Leonard Kelley

Comments

Leonard Kelley (author) on October 11, 2015:

Thank you for the comment Reynold. I appreciate your kind words! I love these topics and am glad that my interest has led to an article you liked.

Reynold Jay from Saginaw, Michigan on October 11, 2015:

Lots of good and fascinating history here! I purchased Carl Sagan's Cosmos earlier this year and then the updated version. I was a science teacher and stuck pretty much to solar system science. Anything beyond that was a bit much for the students. Cosmos does an episode( as you would anticipate ) on Kepler and that is about all I know. I'm happy to see someone like you doing these important articles.

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