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.

A Brief History of Radial Velocity

All radial velocity detections hinge on one thing: that the star is orbiting some center of mass in a system. Isaac Newton theorized that it is not planets which go around a star, nor stars which go around planets, but rather, both going around a shared center of mass. Thus, the star’s “wobble” about the C.O.M. is indicative of a planetary companion. 

The ideal conditions to observe this would be when our line of sight is “edge-on” of the planet’s orbit, since the Doppler shift of light as the star moves depends only on the velocity of the star along our line of sight. In other words, the optimal orientation would be an inclination angle of 90 degrees.

If we assume the orbit of the star is perfectly circular and edge on, simple Newtonian physics yields the amount of velocity we expect to see:


 

To generalize this for all inclination angles, we add a term to account for the projection of the star’s Doppler-shifted velocity along our line of sight (and we can also approximate that the star's mass is much greater than the planet's) to get:




From the radial velocity detection technique, we can determine the planet’s minimum mass. Combined with the transit technique, in which we discover the radius of the planet, we can derive the planet’s average density, and from that can come surface gravity and existence of atmosphere.

*Side note:* Planet masses are usually given a minimum bound by the term “Msin(i).” This is because often, from using the radial velocity technique, we don’t know the inclination angle (i) of the planet’s orbit.

The Beginning of an Era: First Discoveries

Fast forward about 250 years from Newton. In the mid-1900s, worlds beyond our solar system had yet to be probed despite age-old foundations provided by scientists like Kepler and Newton. While standing on the shoulders of these giants, the myriad worlds beyond our own were still mere speculation. We had no idea if exoplanets even existed.

Another renaissance was necessary--another rebirth of inspiration in the field of exoplanetary science to coax the field’s vision into reality. This came in the form of Otto Struve, who estimated there were as many as 50 billion exoplanets and a high possibility of extraterrestrial life.

“[I]t is probable that a good many of the billions of planets in the Milky Way support intelligent forms of life. To me this conclusion is of great philosophical interest. I believe that science has reached the point where it is necessary to take into account the action of intelligent beings, in addition to the classical laws of physics." - Otto Struve

Struve’s 1952 paper was entitled, “Proposal for a Project of High-Precision Stellar Radial Velocity Work” and was only a page and a half long, but was far ahead of its time. It detailed the possibilities of detecting a planet by observing a host star’s motion about the system’s center of mass, and also mentioned “eclipses,” which we now refer to as transits. The paper reasons through why radial velocities should be observable, and concludes that it should be possible to detect Jupiter mass planets, even with the technology of the time. In 1973, Griffin and Griffin published an inspiring article describing a technique that made use of telluric absorption lines to improve radial velocity precision into the regime of planet detection. These conceptual and practical advancements set the course for exoplanetary discoveries later that century, and the philosophical musings that cost Giordano Bruno his life began to enter into the realm of irrefutable scientific confirmation. We were getting closer, and the hunt for planets intensified.

But the search was struggling; all claims of planets found were withdrawn as false alarms. In 1988, Campbell et al. published a paper describing several potential stellar companions, but their data was not concrete enough to earn them the honor of having discovered the first exoplanet. They retracted their claim of planetary discovery. (Though fourteen years later, they were found to have been correct.) Finally, in 1992, a breakthrough was reached in the most surprising of places.

Alexander Wolszczan and Dale Frail were radio astronomers, not particularly looking for planets, but they happened to find not one, but two, around a pulsar. Now these were very different from the Solar System planets, the only known planets of the time. A pulsar is a spinning neutron star, the remnant of a giant supernova explosion, and any planets around one would be formed in a drastically different way than those in our own Solar System. As the first planets detected, it is safe to say that nobody expected them to be like this. Pulsars are observed to have a focused beam of intense electromagnetic radiation, like a spinning lighthouse, and their extremely stable rotation translates to very precise regularity in the arrival time of these radiation pulses. But when the star has a planetary companion, its rotation about the system’s center of mass creates a slight, but detectable, sinusoidal variation in the pulse timing. After combing over the data carefully, Wolszczan and Frail concluded that they had detected exoplanets.

The scientific community went wild.

“We were not looking for planets, we were just a couple of clueless radio astronomers trying to understand a puzzle with the data,” Dr. Frail says, reminiscing upon the time when the paper was published, and the first exoplanets became official. “Our effort gave the field a push.”

The existence of pulsar planets is extremely rare, with only two known systems and a pending third. In terms of studying them, there’s not too much that’s applicable to our solar system, or others. But what this discovery indicated was not a huge opportunity for capitalizing on pulsar planets themselves, instead, it was that the field was “on the right track.”

Enthusiasm and motivation was revived in the search for exoplanets, and from the discovery of the pulsar planets, we entered the home stretch. Although pulsar planets could not be used as a close analogy to our own planet and origins, simply because of their extremely rare nature and strange method of formation, they indicated the existence of extrasolar planets. Three years later, the first exoplanet around a main sequence star was triumphantly detected, and since then, we have confirmed the existence of approximately 786 planets around stars other than the Sun. If it hadn’t been for Wolszczan and Frail, hope and interest may have continued to fizzle away, and knowledge of these new worlds may have remained hidden away in the cosmos.

Many thanks to Dr. Dale Frail for graciously taking the time to interview with us!

xkcd.com

Newton's Cannonball

A deafening boom. A rush of air. The smell of gunpowder. And suddenly a cannonball was arcing through the air. This hypothetical cannonball, launched by Isaac Newton in a thought experiment, could be considered the shot-heard-'round-the-world heralding the arrival of the theory of gravitation.

Newton reasoned if one shot a cannonball off the top of a mountain at a relatively low velocity, it would travel some distance and fall back to Earth. Faster velocities would mean the ball travels farther before it hits the ground. But of course, Newton did not stop there. What if the radius of the ball’s trajectory curve matched the radius of the Earth? The ball must keep falling, but it would never reach the ground. Essentially, it would orbit. So technically, we are falling around the sun, while the moon falls around the Earth! This particular thought experiment that led to Newton’s theory of gravity was only made known after his death. He published very few works, though they were in high demand by some scholars; most of his writings were released only posthumously. But those he did publish, like the enduring Principia in 1687, sent waves through the scholarly community.
Though there were only a few hundred copies in print, Principia could be heard discussed in coffee-houses across England. It held so much information on so many different arenas of science and mathematics, that Marquis de l'Hopital declared of Newton, “Does he eat and drink and sleep? Is he like other men?”

Indeed, Newton was not like other men. As a teenager, he was constantly inventing curious contraptions, such as a mouse-powered miniature windmill, and doodling sketches of animals, people, and shapes on the walls of his boarding home. It says something of his brilliance that in grammar school, he was never taught natural philosophy and only basic forms of arithmetic and math, and yet approximately four years later he discovered calculus! He was raised in a relatively well-off economical situation, but his life was devoid of human connection. Raised by his grandmother, his father passed away before his birth, and his mother left her only son for a new husband many years her senior. In grammar school, the other boys were everything but friendly--likely put-off by Newton's unusual intellectual superiority. Lacking friends at such a young age, he devoted all mental faculties to the pursuit of sciences, and having been scorned by his peers, he carved his name into every school bench he occupied. Later in life he would suffer from several breakdowns, causing his contemporaries to question the soundness of his intellectual abilities. Newton was thus an enigma to those around him. His wildly creative nature was incomprehensible to many, leaving him quite alone.

Through all this, Newton found solace and inspiration in God. While working on Principia, he and one of his few friends, John Locke, would exchange letters, in which Newton would dissect various Biblical passages and their meaning. Newton’s view on the physics of the universe, similar to Kepler’s, were strongly connected to his devout, yet unorthodox Christianity. He would point to God as the answer to many cosmological curiosities, such as how "dark matter" (planets, rocks) was so distinctly separated from "light matter" (the sun, stars): "I do not think explicable by mere natural causes but am forced to ascribe it to the counsel and contrivance of a voluntary Agent."

 

Surviving through an emotionally turbulent childhood, Newton first studied mathematics until the Great Comet of 1680 turned his eyes heavenward.  Johannes Kepler's existing three laws of planetary motion provided a basis and reference for Newton to postulate his own three laws of universal motion.  Newton set off to discover the force which consistently drew objects downward, toward the center of the Earth. The theory of gravitation was, consistent with the famed legend, born out of a contemplation of the trajectory of a falling apple (though the apple never actually hit him on the head), which later spawned the cannonball theory. And, if gravity worked on the Earth, why shouldn’t it be applied to the moon and the rest of the cosmos? Newton declared that the heliocentric model was in fact correct, with one caveat. It was not the Earth that revolved around the Sun, nor the Sun that revolved around the Earth, but rather, both revolving around the system's center of mass. In his own words, "the common centre of gravity of the Earth, the Sun and all the Planets is to be esteem'd the Centre of the World."

For all his eccentricity, Newton died quietly in his sleep in 1727. His equations for the motions of the planets and other celestial bodies are crucial foundations for exoplanetary science. In addition, his stipulation that components of a system orbit about a center of mass is the cornerstone of the radial velocity method of detecting planets. His crowning achievement is his theory of gravity, and if you were to fire a cannonball from the top of a mountain today, you’d find that the results would not have changed from Newton’s original postulations. However, almost 200 years later, gravity would be radically expanded to address high energies and speeds, permanently changing our perspectives on astrophysics by a German man with disheveled hair, who would come to be known as the father of modern physics: Albert Einstein.

“I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”

Isaac Newton