* While it long seemed plausible that there were planets around other stars, it wasn't until the last decade of the 20th century that the technology became available to detect their presence. By the beginning of the 21st century, dozens of extrasolar planets were known and the list was continuing to grow in both length and detail. This chapter outlines the history and current state of the search for extrasolar planets or "exoplanets".
* Once astronomers realized that the stars were distant suns, they immediately began to wonder if there were planets orbiting them, as they do around our own Sun. Many astronomers believed that was probably the case, but detecting planets around other far-away stars was simply beyond the technical capabilities of astronomers, at least until the 1990s.
Since 1995, planets have been detected around dozens of other stars. This was an amazing accomplishment. Directly spotting a planet around a distant star is very difficult, since the planet is relatively small, emits little or no light of its own, and is drowned out by the glare of its nearby parent star, a situation something like trying to use a telescope to spot a gnat circling around a searchlight.
These exoplanets were found by mapping the slight variations in the motions of the star caused by the gravitational pull of a planet or planets. These variations are revealed by very small changes in the wavelengths of light from the star, known as "Doppler shifts", due to the star's motion along the line of sight to Earth. The star's wobble occurs because two bodies in space linked by gravity actually orbit each other, instead of one orbiting the other. If two planets of the same size are in such a mutual orbit, they will both trace out an orbit around a point halfway between them. As one body gets bigger than the other, the center of the mutual orbit moves closer to the bigger body. If the difference in size is large, the center of the orbit may be under the surface of the larger body, but it will never be precisely at the center.
For example, the solar system's biggest planet, Jupiter, has a thousand times less mass than the Sun, but it still has a clearly measurable influence on the position of the Sun. Jupiter completes one orbit every 11.8 Earth years, and in that time, the Sun will shift around in a circle a thousand times smaller. The Earth's mass is 318 times smaller than that of Jupiter, and its influence on the Sun's motion is much less noticeable, though magnified by the fact that the Earth is five times closer.
The Doppler shifts caused by the presence of a giant planet around a distant star amount to only about one part in ten million. To find exoplanets, astronomers have developed techniques to detect changes in frequency amounting to one part in a hundred million. This trick is performed using an "iodine absorption cell". This is a glass bottle full of iodine gas that is placed near the focus of a telescope. Iodine gas absorbs certain specific frequencies of light, and absorption from the cell leaves dark "absorption lines" in the spectrum obtained from the star. The iodine absorption lines can be used as a precision reference to measure the Doppler shifts in the stellar spectrum.
Despite the amazing precision of Doppler measurements, the wobbles of stars are so subtle that the Doppler search method is limited to a range of about 50 light years. In addition, Doppler measurements can only give the minimum mass of these planets, with the actual mass as much as twice as big. Unless astronomers observe a star exactly edge-on to the orbital plane of its planets, which is not very likely to happen, then they will see the Doppler shifts only of the component of the motion in that plane, and the mass of the planet will seem smaller. In most cases, there is no way to determine the orientation of the orbital plane.
* Occultations or "transits" of a star by one of its planets have also been used to detect exoplanets. For such a transit to be observed, the plane of the planet's orbit must be very near to edge-on to the line of sight from Earth; there is about a 10% chance that it will happen for a planet of Jupiter mass or larger. Such an event can be detected by measuring the slight decline in the star's light output during the transit. A transit will reveal the planet's orbital inclination, and also provide an estimate of its actual diameter. The scheme can be used to spot smaller planets than can be picked up by Doppler measurements.
A conceptually similar but even more difficult technique, called "gravitational microlensing", is also being used for exoplanet hunting. Gravitational microlensing involves the short-lived distortion of the image of a distant star when some nonluminous body, usually not near the star, passes through the line of sight to Earth. The problem is that such events are rare, and it requires an automated telescope system performing a continuous methodical search to find them. Such a search, named the "Optical Gravitational Lensing Experiment (OGLE)" has been running for a number of years, scanning the central regions of our Galaxy every night.
However, OGLE's primary rationale is to determine the number of nonluminous objects in our Galaxy. OGLE doesn't revisit the site of a microlensing event often enough to hunt down an exoplanet, but as of late the project has been modified to pass down the coordinates of an event to other automated telescope systems for more careful observation. The scheme has proven workable, discovering a candidate planet 5.5 times the mass of Earth about 20,000 light-years away. Advocates believe that it can spot planets only a tenth of the mass of Earth. More powerful automated telescope systems are now being introduced to increase the effectiveness of gravitational microlensing searches.BACK_TO_TOP
* The burst of exoplanet discoveries began in October 1995, when Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland performed Doppler measurements of the star 51 Pegasi, in the constellation Pegasus, and discovered a very slight wobble in its motion across the sky. The data collected by the two astronomers indicated the presence of an unseen companion that was too small to be a star, orbiting 51 Pegasi once every 4.2 days.
Following the Swiss finding, a survey of 107 stars similar to our own Sun, performed by astronomer Geoffrey Marcy at San Francisco State University and the University of California at Berkeley, R. Paul Butler of the Carnegie Institution in Washington DC, and their colleagues discovered six more planets, one of which was independently discovered by other astronomers at the same time. An eighth planet, orbiting the star Rho Coronae Borealis, was discovered by a Harvard University team in April 1997. In late 1999, astronomers working with Marcy's group and using the Keck telescope discovered a half-dozen new exoplanets; an automated telescope system in Arizona then obtained the first occultation event of an exoplanet with the star HD 209458.
Although the first exoplanets to be found by Doppler search were discovered in 1995, in 1992 a radio astronomer named Alex Wolzczan performed an analysis of the variations in the timing of a "pulsar", a superdense neutron star that emits regular radio pulses as it spins. Wolzczan determined that it was orbited by two Earth-sized and one Moon-sized planet. This was a remarkable accomplishment, but not as widely applicable as the Doppler search method, and Wolzczan's discovery has become something of a footnote to the later discoveries of exoplanets.
Planetlike objects estimated to be from 5 to 15 times the mass of Jupiter were also directly observed in a young star cluster in the center of the constellation of Orion in 2000. These "free floating" objects, not associated with any star system, were spotted in the Sigma Orionis cluster by astronomers of the Astrophysics Institute of the Canary Islands through telescopic observations in the infrared. While the observations have been confirmed, some controversy remains over the size estimates, which were based on models of the cooling rates of cosmic bodies as a function of mass. As a result, these objects remain even more speculative than the exoplanets discovered by Doppler searches.
As of 2016, over 3,400 exoplanets in over 2,500 star systems had been identified. Although there have been a few false alarms, the size of the list keeps on growing.BACK_TO_TOP
* The new planets have presented a number of puzzles to astronomers. The classic theory of planet formation holds that the gaseous materials in our Solar System spiraled inward to form a growing Sun at the center of a swirling "protoplanetary disk" of gas and dust. The planets clumped together under gravitational attraction within the disk, while the Sun began to glow with its own fusion processes. Planets couldn't form too close to the Sun because the Sun swept up nearby material and also made it too hot for bodies to coalesce. The disk thinned out at its edges, bounding how far away planets could be formed.
The exoplanets that have been discovered have confounded this scheme. To be sure, astronomers know they don't have a representative sample. The Doppler technique is biased towards planets that are big and relatively close to their primaries. In fact, if alien astronomers have performed the most sensitive Doppler measurements on our own Sun, they might discover Jupiter and possibly Saturn, but they would not be able to detect any other planets.
However, even given that the sample may be unrepresentative, the discoveries have provoked some puzzling questions. Many of the new planets are substantially bigger than Jupiter. That is not surprising in itself, though it seems notable that none are more than ten times bigger than Jupiter. Since the Doppler search technique is biased toward the discovery of big planets, the lack of really big planets suggests that there is some "cutoff" threshold to the size of a planet. The Doppler searches also show that the "brown dwarf" substars, 13 to 75 times the mass of Jupiter, seem to be very scarce, since if they were present in those systems they would certainly be detected.
The real problem with the big planets is that many of them are close, in some cases really close, to their primary star. For example, Mercury's orbital radius is 0.38 AU and its orbital period of 88 days. However, the planet orbiting 51 Pegasi has an orbital radius of about 0.052 AU and a period of 4.2 days.
The classical theory of planet formation suggests that big planets are formed at much greater distances from the primary star, since the heat of the star during the formation of the stellar system would have otherwise prevented materials from accumulating to form such a big planet. In reality, some of the "51 Pegasi" planets, as the big and close exoplanets are now known, are several times bigger than Jupiter.
New models have been devised in which these big planets were indeed formed at much farther distances from the primary star, but then migrated inward due to the drag of the planetary disk. The problem with these models is that the planets should have ultimately been pulled into the primary star and absorbed. Theorists have been able to propose several reasons why this didn't happen. One notion that several planets were absorbed in this fashion, but those that survived achieved something like their current orbit just as the planetary disk, which only lasts about ten million years, was finally dissipating, and so the planet did not fall inward any further.
Even if the planetary disk had not dissipated, magnetic fields from the star could also clear a gap out of it, and the planet's orbit would no longer decay. This would only happen in regions very close to the star, but tidal forces could transfer angular momentum from the star to the planet, causing its orbit to move back outward.
Such giant planets in close stellar orbits would be subjected to such a blast of heat and energy that they would lose most or all of their atmospheres and be reduced to cores of glowing or melted rock. Astronomers have named such worlds "cthonians" -- "hell planets".
The other difficulty that the exoplanets pose for theorists is that those that aren't in close orbits have very eccentric orbits. The "classical" theory suggests they should be more or less circular. Astronomers have proposed models that if a number of very large planets are formed in a star system, they may interact in such a way as to eject some of them into interstellar space and leave the survivors in highly eccentric orbits. Marcy and Butler have compared this sequence of actions as something like the "break" in a pool game that scatters the balls over the table.
* One astronomer, David F. Gray of the University of Western Ontario in Canada, has suggested that some of the new planets are so preposterous that they cannot exist, and that the wobbles in the stars are due to stellar oscillations, not to an unseen companion. The planet hunters went back to refine their data and argued against Gray's theory. Stellar models suggest that such oscillations would occur at higher frequencies than those observed, and in any case oscillating systems normally oscillate on multiple frequencies, consisting of a strong oscillation at a low "base" frequency and weaker "harmonic" oscillations at integer multiples of that frequency. Harmonics have not been observed. Oscillations also imply brightness changes, and these have not been observed, either.BACK_TO_TOP
* While the new discoveries have upset traditional ideas about planet formation, it may be difficult to come up with a persuasive model to replace the old one until astronomers track down smaller planets and obtain a clearer knowledge of the organization of exoplanetary systems. Sky surveys have been increasing the number of candidate planets down to the point of diminishing returns; long-term observations are also providing data to help find planets with longer periods than those discovered to date, and refining what we know about exoplanets already discovered. However, to find the smaller planets, new technology will be required.
Improved ground-based measurements are now available using optical and near-infrared interferometer systems, such as the twin ten-meter Keck Telescopes in Hawaii and the European Southern Observatory's Very Large Telescope (VLT) array of four eight-meter telescopes in Chile. These interferometers combine beams of light from several large telescopes to provide Doppler measurements with ten to a hundred times greater sensitivity than previously available, permitting the detection of smaller and more distant worlds.
A ground-based telescope dedicated to planet hunting came on line at Mount Hamilton in California in early 2014. The Lick Observatory's "Rocky Planet Finder" AKA "Automated Planet Finder" features a robot telescope with a 2.4 meter mirror and a spectrographic system optimized for hunting planets via Doppler shifts. There are also ground-based occultation observatories, such as the "Next-Generation Transit Survey (NGTS)", an array of twelve 20-centimeter telescopes, sited in the high Atacama desert of Chile. It was built by a consortium of British and Swiss universities, along with the German space agency DLR.
Several space missions focused on checking for planetary occultations and observing stellar oscillations (to help determine their internal structures) have been launched or are in planning. The Canadian "Microvariability and Oscillations of Stars (MOST)" satellite was launched along with a set of other payloads by a Russian Rockot booster from the Russian Plesetsk cosmodrome on 30 June 2003. MOST was a suitcase-size astronomy platform carrying a telescope with aperture of 15 centimeters and a sensor system capable of detecting variations in stellar brightness of only 1/10,000th of a percent. It was a relatively low-cost mission, but it was Canada's first space astronomy satellite, and it paved the way for bigger missions.
The French launched a space observatory named "COROT (Convection, Rotation, & Planetary Transits)" on a Russian Soyuz Fregat booster on 27 December 2006, the mission ending in 2012. It searched for planetary occultations, and also performed detailed measurements of stellar oscillations to obtain a better understanding of their internal structures.
NASA launched a more sophisticated stellar occultation mission named "Kepler" with a Delta 2 booster on 6 March 2009. Kepler had a 1-meter telescope with an imager featuring a resolution of 95 megapixels. The spacecraft was placed in an Earth-trailing solar orbit so it would not be obstructed as it orbited our planet. Although Kepler had an upset in the spring of 2016 and went into emergency mode, ground controllers managed to get it back into operation. However, it has to babied to a degree to get it to work, since by the time of the upset, it had lost two of the four reaction wheels used to reorient the spacecraft. Kepler has, to date, discovered two thousand exoplanets, more than all other exoplanet searches combined.
As a follow-up to Kepler, NASA now plans to launch the "Transiting Exoplanet Survey Satellite (TESS)" in 2017 to continue the hunt for exoplanets during a two-year mission. TESS will have a launch mass of about 325 kilograms (717 pounds), with a payload of four wide field-of-view 16.8 megapixel cameras to cover 400 times the area of the sky seen by Kepler.
TESS will monitor 500,000 stars across the entire sky, watching for temporary dimming caused by the passage of a planet between the star and Earth. TESS will be launched by a Falcon 9 v1.1 booster from Cape Canaveral. It will be placed in a highly eccentric, high altitude Earth orbit, in a resonance with the Moon's orbit, ranging from 108,000 to 374,000 kilometers (67,000 to 232,000 miles) on a period of 13.7 days.
Over the longer run NASA is interested in actually observing exoplanets, having proposed a large space observatory tentatively named the "Terrestrial Planet Finder (TPF)". The TPF would take pictures of exoplanets the size of Jupiter or larger, and obtain their spectroscopic signatures to determine atmospheric compositions. A planet with an oxygen atmosphere would be very likely to harbor life, since reactive oxygen would otherwise form oxide molecules and be removed from circulation.
As mentioned, actually imaging an exoplanet is tricky because the central star overwhelms the image. The trick is to eliminate the light from the star while leaving the light from everything around it. Interferometers can perform what is called "nulling", in which the light from the central star is subtracted from itself to cancel it out. NASA considered an interferometer configuration for TPF, with the optical elements mounted on a fold-up truss structure, or as individual spacecraft flying in a precise formation. A second option would be to build TPF as a "coronagraph", with a large single elliptical mirror featuring "pupil masks" that create a crisp optical boundary between the star and its surroundings. A third scheme is the "vortex phase mask", a glass plate etched in a spiral or concentric ring pattern that is placed before the imaging system of a telescope to perform the nulling.
TPF has not been funded, and it amounts to a shelved concept for the time being. The ESA has considered a similar project named "Darwin", but no schedule is available for its launch, either. It is possible that NASA and the ESA will eventually merge their efforts into a single collaborative mission.
The discovery of exoplanets has posed many more questions than it has answered. We now have strong evidence that such planets exist, but the structures of exoplanetary systems hint that our system of eight planets in circular orbits may be unusually orderly. If highly eccentric orbits are more the norm, conditions on most planets may vary between extremes, making conditions for life more difficult. The only way to know for sure is to look harder.BACK_TO_TOP
* I never particularly planned to write anything as extensive as EXPLORATION OF THE PLANETS. Like many of the projects I work on, I just wanted to write some notes to organize my thoughts, and it got out of control.
I'm not a particular sky-watcher, though I will poke around a bit with binoculars every now and then. I've had telescopes and usually sold them off because they gathered dust. It's nice to see Saturn or the moons of Jupiter but not anything I would spend a lot of time, effort, or money to do.
I have found comets interesting. I saw my first comet, Comet Kohoutek, when I was stationed down in Texas with the US Army in 1974. There was considerable advance publicity for Kohoutek, since it was a big comet that apparently hadn't visited the inner Solar System before, but it didn't generate a big tail and was generally regarded as a bust, "the comet unseen by billions". That was somewhat unfair, since I clearly remember seeing it in the evening, rivaling the "evening star" Venus in brightness, which made it substantially more than some piece of mere sky junk. However, as a result of big letdown, when the next big comet came by the astronomers didn't make a fuss over it.
One wintry morning in 1976, when I was living in Spokane, Washington, I took the dog out for a walk in the darkness, only to see a ghostly apparition streaking up from the horizon ahead of the sunrise. It was quite a sight, though I was so indifferent to the specifics that I didn't learn for years that the object was Comet West 1976 VI, with a period of half a million years, making it definitely a "mysterious traveler" from the deep reaches of the Oort Cloud. It broke up into multiple pieces during its transit. Astronomers hadn't made much of a public fuss about it, having been embarrassed by the failure of Comet Kohoutek to put on a show.
Twenty years later, in 1996, I was fortunate to see another spectacular naked-eye comet, Comet Hyakutake. Its path across the northern regions of the sky made it extremely easy to find. I first spotted it on Friday, 22 March 1996, as a fuzzy blob very roughly twice the angular size of the full Moon, as best as I could eyeball it, covering the bright star Arcturus.
I scoped it out again as it fell across the Little Dipper on Monday the 25th, the day of its closest approach, about 16 million miles away above the North Pole, so I am told. Either by a change in viewing angle or in the comet's behavior, it seemed to have gone from fuzzball to the more traditional idea of a comet, with a long tail streaming away from the Sun. The increasing presence of the Moon over later nights made observing more difficult. I can see why in ancient times comets were regarded as bad omens. Their fuzzy appearance makes them somewhat eerie and their rapid transit of the sky, moving visibly from night to night, seems like an unnatural disturbance of the otherwise unchanging skies.
* As of the v1.3.1 release, I changed all references of "manned spaceflight" to "crewed spaceflight". That was not without misgivings; there was no good reason to cling to the old phrase, but the new one has an awkward sound to it -- "crude spaceflight"? However, the fact was that the phrase "manned spaceflight" simply had to go. One of these days, someone may come up with a cleaner phrase than "crewed spaceflight" -- then again, maybe we'll just get used to it.
* As concerns copyrights and permissions for this document, all illustrations and images credited to me are public domain. I reserve all rights to my writings. However, if anyone does want to make use of my writings, just contact me, and we can chat about it. I'm lenient in giving permissions, usually on the basis of being properly credited.
* Sources include:
Of course, a number of websites were consulted as well as print sources. One of the most important was Bill Arnett's excellent "A Multimedia Tour Of The Solar System", which is not only readable and thorough in itself, but provides an extensive set of links for investigation down to excruciating detail. The NASA, ESA, and JAXA websites were also consulted.
* Revision history:
v1.0.0 / 01 feb 02 / Released as EXPLORATION OF THE PLANETS. v1.1.0 / 01 feb 04 / Overall update, expanded by 2 chapters. v1.1.1 / 01 apr 04 / Rosetta launch, quick fix of nasty typos. v1.1.2 / 01 aug 05 / Usual sort of "stay current" updates. v1.2.0 / 01 jan 07 / Rearranged comet & Pluto chapters, updated. v1.2.1 / 01 mar 07 / Cleanup, changed title to "MISSIONS". v1.2.2 / 01 feb 08 / Various updates and trims. v1.2.3 / 01 jan 09 / Illustration clean-up & various minor updates. v1.3.0 / 01 apr 10 / Review, polish, and update. v1.3.1 / 01 oct 10 / Review, polish, and update. v1.3.2 / 01 sep 12 / Review, polish, and update. v1.4.0 / 01 aug 14 / Additions, review, polish, and update. v2.0.0 / 01 jul 16 / Expanded to 20 chapters from 19, many new probes.
Except for the materials on the Moon and exoplanets, the chapters in this document were originally released as stand-alone documents in the following sequence:
jun 00: The Exploration Of The Asteroids nov 00: The Exploration Of Mars dec 00: The Exploration Of Pluto apr 01: The Exploration Of Mercury jul 01: The Exploration Of Venus aug 01: The Exploration Of The Comets sep 01: The Exploration Of Neptune oct 01: The Exploration Of Uranus nov 01: The Exploration Of Jupiter dec 01: The Exploration Of Saturn jan 02: The EarthBACK_TO_TOP