Monday, August 13, 2012

Hitting the Gold Mine

The past twenty years of exoplanetary science has been somewhat of a treasure hunt. The focus has been on finding planets, of all different shapes and sizes, using all different methods, searching high and low and examining countless stars. Emotions and excitement were high, as the race to the treasure trove intensified. Some efforts were a bust, like the study of Walker et al. (1995) in which no planets were found in fourteen years of data, and there were many tentative announcements of potential exoplanetary discovery that just didn’t have solid enough data, such as Campbell and Walker’s in 1988. Several papers were published during the late 80s and early 90s claiming planet discovery, but almost all were quickly retracted. In 1991, a year before Wolszczan and Frail published their landmark paper, Andrew Lyne and his team claimed to have found a planet around a pulsar, but almost immediately retracted it. So common was the fool’s gold--false positives and withdrawn claims--that the first real exoplanetary discovery was met with high skepticism.

In 1992, Tutukov estimated that 30% of all stars had planetary systems. Although this was an estimate and the paper only has two references to date, it is a testament to the expanding possibilities and anticipations of discovering more planets, perhaps some even like ours. Early detection methods were occasionally extremely dangerous; some required using toxic hydrogen fluoride cells to provide absorption reference lines to calibrate wavelengths of starlight. This was Campbell and Walker’s main tool for their 1988 paper in which they published many potential planet candidates. It seems that luck just wasn’t on their side, for though they are not considered the first exoplanet discoverers, twelve years later it was proved that one of their candidates was indeed a planet; perhaps the risky methods they used had paid off in the end. Using HF later evolved to safer methods, and in 1992 Marcy & Butler used iodine absorption cells to make high precision radial velocity measurements.

Several teams of extremely capable scientists were neck-and-neck in the hunt. Controversies arose as these teams raced to announce their findings, and it’s interesting to note that there are several cases of milestone papers being published very close together, sometimes even in the same month and issue of a journal. The discovery of the first exoplanet around a sun-like star was a very close announcement in which Mayor et al. edged out Marcy et al. by about six months. Marcy and his team had the 51 Pegasi data on their hard drive at the time, but simply lacked the computer resources to process it. Regarding the first exoplanetary transits observed, Charbonneau and Henry’s groups both published their papers in January 2000 edition of the Astrophysical Journal. This trend continued with Marois et al. and Kalas et al. in their publication of papers detailing directly imaged exoplanets.

When the gold mine was finally discovered, the door was wide open to discovery after discovery of treasured exoplanets. Planets were found left and right, and new ways to discover them were added to the standard radial velocity method. (Had Tutukov’s paper not been overlooked, perhaps the first detected planet would have been found using the transit method.) In 1991, detection by the gravitational microlensing method was proposed by Mao & Paczynski, but it wasn’t actually used to detect an exoplanet until 2002. The method is made possible by Einstein’s theory of general relativity, utilizing its esoteric concepts of spacetime curvature. It requires photometric data like the transit method, but unlike transits, using this technique we detect an increase in a star’s luminosity. This is attributed to light bending around a planetary companion, the planet focusing the light like a lens.

In 2004, we succeeded in actually “seeing” an exoplanet using the direct imaging technique. This is exactly what it sounds like: taking a direct “picture” of an exoplanet. This technique has discovered about half a dozen planets. As one could assume, since exoplanets are so faint and tiny (the planets around HR 8799 are 25,000 times dimmer than their host star!) it relies heavily on instrumentation. The process requires blocking out the starlight in order to see dimmer companions. Direct imaging allows us to take a spectrum of the planet and determine its properties, and unlike transits and radial velocity measurements, it is not limited to finding planets with close-in orbits. In fact, direct imaging is most sensitive to planets in wide orbits, and through its use our zoo of planets diversified.

The method of astrometry was used to characterize an already-discovered exoplanet in 2002, but wasn’t actually used to detect a planet until 2009. Astrometry involves measuring a star’s path across the sky and whether that path indicates a planetary companion. This is similar to the radial velocity method of detecting motion about a center of mass.

By the time the Kepler mission came around and astonished the world by its discovery of thousands of potential planets, approximately 90% of which are real, finding planets wasn’t such big news anymore. And now we come to the present, where simple discovery isn’t enough, but before us lies a whole universe of promise. The same excitement in the beginning still exists, but directed towards a different question: where will the exoplanetary field go from here?

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