Future Prospects

Astronomy is the one of the few sciences in which our subject is indirectly studied. We cannot deal hands-on with stars, planets, or nebulae; our only tools to explore the vast cosmos are the lenses that we look through, and thus our picture of the universe depends solely on how good our eyes are. And so astronomers are meticulously counting photons, de-convolving images, continually calibrating instruments and developing new and innovative technologies. We can’t, nor would we want to, hold a piece of a star; all we can hold are images.

However, human ambition and curiosity are not to be limited. Within the last hundred years, we have gone to the moon and Mars, sent space probes out of the Solar System, and discovered countless exoplanets. We have an intrinsic desire to see up close what we are investigating; there is currently a proposed mission (you can see for yourself if it’s believable or not...) to send people to inhabit Mars by 2023. It seems that an inevitable desire in human nature is expansion.

The exoplanetary discovery missions thus far have discovered a zoo of exoplanets. Gas giants, rocky terrestrial planets, and others line the spectrum, including ones in the nebulously defined habitable zone of their host star, in which liquid water can exist. We are learning that there are probably many pale blue dots out there, and perhaps even other life. The degree of complexity life may hold across the cosmos, we don’t know until we find it. Now comes the speculation. We stand on the precipice of more fascinating discoveries, looking ahead to the future. Will we actually visit another planet? Will we find life anywhere else in the cosmos?

It’s kind of like groping around in the dark. As every scientist young and old knows, experiments must be performed several times in order to get accurate results, but the Earth has only given us one grand experiment to work with. Our definition of life is limited by our own experiences here on our pale blue dot; we could be as wrong as Aristotle’s geocentric model of the universe. And perhaps “life” is in fact everywhere in our universe, but we are desensitized to it by our constraining definition.

The future prospects are extraordinarily exciting. But noble ideas of expansion and exploration can sometimes eclipse our present realities; sometimes we are so preoccupied looking through our lenses at the sky that we are blinded to what is here on the Earth. There is much potential here at home. While discovering other planets like ours, we still have a responsibility to care for, to “preserve and cherish” in the words of Carl Sagan, our own pale blue dot and the life that exists on it.  While the pursuit of knowledge for its own sake is an excellent thing, should creating settlements on Mars be a priority when our own housekeeping may not be complete?

The coming years for exoplanetary science will give us worlds of knowledge about our universe and our place in it. The potential exists to bring people together from across the planet. Having this cosmic perspective should be incredibly humbling and eye-opening to how thankful we should be to live and think on our little speck in this dangerous universe. A cosmic perspective should be equalizing, showing us that no matter what social stratifications may exist, we all ultimately are in the same position. Though we can’t actually hold the stars and touch the galaxies, in the words of Neil DeGrasse Tyson, “The atoms of our bodies are traceable to stars that manufactured them in their cores and exploded these enriched ingredients across our galaxy, billions of years ago. ... We are not figuratively, but literally stardust.” And perhaps, in our quest to understand ourselves, we will find other living beings made up of stardust, just like us.

“We are now creations of the universe standing upon a floating stone, pieces of the universe taking the first steps toward understanding itself.” - J. Swift

The Kepler Mission

It seems the name “Kepler” carries with it a certain weight of great discovery. First Johannes Kepler, who revolutionized conceptions of the laws that govern the “celestial spheres” in our night sky, and then his name was passed onto the Kepler mission, which has, as of August 17 2012, discovered 2321 planet candidates (the fidelity of this sample was calculated by Exolab’s Tim Morton here). Only 20 years ago, the existence of exoplanets was still a hazy theory.

The Kepler mission has been specifically designed to focus on one area of the galaxy and find Earth-sized terrestrial planets in the habitable zones of their host stars using the transit detection method. It simultaneously measures starlight from more than 100,000 stars in its field of view! From the transits that Kepler detects, we can use the laws that Kepler devised to determine orbital and planet sizes. During its lifetime, it will use the information it collects to calculate exoplanetary temperatures, make estimates of planetary size and mass distributions throughout the Galaxy, and learn more about planets in multiple star systems. In addition to putting together pieces of the planet formation puzzle, we will then have a better picture of our own context within the planetary system spectrum in this Galaxy.

Like any new technology or mission, Kepler came with many other unforeseen benefits. It gave new life to the field of astreoseismology, as the ultra-precise light curves from Kepler allow us to measure the minute pulsations of certain stars. This gives us independent measurements of the density, mass and age of these stars. Much of what we know about planets comes from our knowledge of their host stars, and thus this seemingly tangential science pursuit is completely relevant to planet-finding.

Kepler's field of view. From www.kepler.nasa.gov

All technical details aside, scientists hope that the Kepler mission will give us insight into properties of planets that may be similar to ours, and perhaps into our own origins. The sheer act of finding thousands of terrestrial planet candidates is philosophically astounding: the paradigm shift is drastic, and we have come so far from thinking we are the center of the universe.

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?

The Study of Light: Transiting Planets

Distances on an astronomical scale are somewhat incomprehensible to us as humans. Currently, there’s no feasible way we can personally go anywhere farther than perhaps Mars, all we can send as a human representative is some kind of unmanned robot. (As a side note, Curiosity has landed!) In order to study the vast depths of space, we’re relatively limited to one of our senses, our eyes. Studying astronomy is essentially detecting and analyzing photons: for now, all we can do is measure light.

But what we can infer and learn from that light is astronomical on its own scale. From stellar light, we can learn about stars’ chemistry, size, distances, and the list goes on. One of the most recent and exciting developments in our knowledge of outer space is something that has been theorized for centuries--the existence of extrasolar planets--and comes simply from observing light.

Just like when our moon eclipses the Sun, when exoplanets eclipse, or transit, their host star, the star grows visibly dimmer. The method of observing planet transits is centuries old, and originated in our own Solar System. The early transits of the inner planets in our Solar System were predicted by Johannes Kepler, which were generally very accurate. This isn’t surprising, as the transit method hinges directly upon Kepler’s Third Law.

But planets are tiny compared to stars. Observing an exoplanet transit requires very precise instrumentation to be able to detect such tiny fluctuations in stellar flux. However, modern technology has evolved to the point where we can do this, measuring changes up to a part in ten thousand, or better. Thus, when a star has a planetary companion and its orbit is oriented just right, we observe a dip in the star’s light curve. Occultations, sometimes called secondary transits, occur when a planet passes behind its host star and there is a consequent dip in light, due to the planet’s light being lost. However, this is a much smaller effect, and is currently limited to only the biggest and hottest planets.

From transits, we observe three quantities: , where P is period, tau is transit duration, and delta is how “deep” the light curve is in relation to the star’s non-transit stellar flux, a quantity equal to .

The transit detection method has evolved exponentially, simply beginning with Mercury and Venus passing in front of the Sun.

1631: Mercury’s transit was predicted and observed.
1639: Transit of Venus was observed by Horrocks and Crabtree. As a result, the estimated size of the solar system increased ten times, but the estimate was still smaller than the actual size.

A mural at Manchester Town Hall by Ford Madox Brown. From wikimedia commons.

During the mid-1900s, we hadn’t observed an exoplanet transit yet, but this did not mean that scientists didn’t speculate about them.

1952: Otto Struve asserts that during a transit, the observed stellar flux would decrease in magnitude by .02. This corresponds to a fractional change of 1/50. This would have been observable by photometric technology of the time, but none were detected.

1984: Borucki and Summers postulate that in order to detect terrestrial planets, more precision was needed to lessen atmospheric influence. Their proposed program required monitoring hundreds or thousands of stars and expected to detect one planet per year.

1992: Tutukov estimates that 30% of all stars have planetary systems. He said the search for planets around sun-like stars was “unpromising” using the radial velocity detection method, and we should approach finding exoplanets with the transit method. He estimates that for 4000 stars with , ten should have planetary systems detectable by transit. In addition, he postulated that brown dwarfs cannot be detected except by transit.

The first extrasolar transits observed were from exoplanets that had already been detected with radial velocity measurements.

1995: Mayor et al. discover a Jupiter-sized companion to 51 Pegasi from radial velocity measurements. The planet has a semimajor axis smaller than Mercury’s, and is either a gas giant or a brown dwarf. This is the first exoplanet discovered around a sun-like star.

1999: Henry, Marcy, Butler, and Vogt find a transiting 51 Peg-like planet. First radial velocity was used to detect the planet, then the transit was observed--the first extrasolar transit. The planet’s radius, mass, and density were derived to imply a gas giant in a circular orbit.

1999: Charbonneau, Brown, Latham, and Mayor publishes a paper simultaneously with the Henry group, also using radial velocity to find an exoplanet and then acquiring its light curve. It was also a gas giant, with a semimajor axis of .047 AU and signs of water vapor. It was determined to have an atmosphere, and was one of the first to be observed spectroscopically. The team found the planet’s mass, radius, surface gravity, escape velocity, and average density. This planet turned out to be the exact same planet that the Henry group was observing, and therein lies controversy on who actually recorded the first extrasolar transit.

Finally, a planet was actually discovered via transit, giving rise to the mass transit surveys we see today. Simply analyzing photons from a distant star pointed to the existence of a planetary companion.

2003: Konacki et al. discovered the first planet using the transit method. After the radial velocity method was combined with the transit spectra, the planet was found to have .9 Jupiter mass, a semimajor axis of .023 AU, and a temperature of 1900 K (hot!).

2004: Alonso et al. conducted the first transit survey.

2006: CoRoT, which stands for Convection, Rotation, and planetary Transits; is launched in December by the French. Its primary purpose was to use stellar seismology to discern the inner structure of stars, but it also used transits to detect companion planets. It aims to find telluric rocky exoplanets similar to Earth. It does not publish planet candidates, only fully characterized planets.

The COROT Space Telescope. (Copyright 2006 CNES)

2009: The Kepler mission is launched in March, aiming to find potentially the largest sample of exoplanets to date through transit photometry. It is designed to discover Earth-like planets in the habitable zone of their host stars by focusing on one small (but deep) portion of the sky that includes about 150,000 target stars.

Kepler's field of vision in the Milky Way. From wikimedia commons.

Scientific discovery, as seen with the transit detection method, is a lot like a fire. Once the first spark is ignited, it grows, expanding and encompassing. The method of detecting planets via transit originally began as a prediction, but has developed into a reality which has enabled us to learn so much more about planets beyond our solar system, planets around distant specks of light. And the beauty of discovery is that there is no limit; our knowledge of exoplanets continues to broaden, and there will always be fascinating things to uncover.