Brief Project Description

Taking the Measure of Planets in the Globular Cluster 47 Tucanae

Project Description:

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Topics:
Significance of Extrasolar Planets

        Transits, Radial velocity Searches, Photometry
           Transit Method
           Radial Velocity Variations
           Astrometry
           Photometry

Project Justification

Significance of Extrasolar Planets:

The search for extrasolar planets is hardly new. In centuries past, people have wondered at the possibility of other worlds orbiting the stars in the sky. Only in the last decade of the twentieth century has this long search come to fruition. The long wait for the first planet outside the solar system ended in 1991 with Dr. Aleksander Wolszczan's discovery of 3 roughly earth-sized planets orbiting the pulsar PSR B1257+12 (confirmed in 1994). Since then the list of confirmed extrasolar planets has grown to nearly 30, with most of these orbiting stars not unlike our Sun.

There are many benefits to searching for these planets (and many more when they're actually found!). For one, it allows for estimates as to the typical abundance of planets around stars. Certainly not every star will have planets, and even when formed planets can be lost from their parent systems. Some stars probably do not last sufficiently long to allow for planet formation. These stars, many times more massive than the sun, and with lifetimes of a few millions or hundreds of millions of years, progress to quickly through their evolution, and go out in a blaze of glory, long before planets can form from the surrounding disk of gas and dust.

Planet searches also lead to theoretical work concerning formation of planetary systems. Real observations test models and simulations that predict how planets first form from disks of dust and gas around the young parent star. These models go on to show how planets proceed about their orbits in the system. Some systems may remain stable over the lifetime of the star (like our own, fortunately), while other planets suffer more dire fates. Some models predict that planets would migrate inward, spiraling suicidally into the host star as they accrete more mass from the protoplanetary disk. In other cases, planets are ejected from the systems during close encounters with passing stars; some smaller ones are simply kicked out by the neighborhood bully (a large Jupiter-like planet in the same system). In our own system we may owe our existence to the presence of a large gas giant (or two). As Ken Croswell explains in Planet Quest, the presence of Jupiters and Saturns in a system may aid the development of life on inner worlds by deflecting potentially lethal incoming comets.

Many planets found around other stars provide clues as to the formation of certain types of planets at various orbital distances. Do terrestrial (earthlike, rocky) worlds tend to form in the inner regions of a protoplanetary disk? Are jovian (gas giant) planets normally found in the outer portions? New observations continue to challenge many long-standing theories that our own solar system is the archetype for all systems. Planets found so far, however, have been Jupiter-sized and orbiting very close in to the parent star (more on this below).

Perhaps a more "human" connection to these searches is the hope that in discovering a wealth of extrasolar planets, we may one day stumble upon a world much like our own. Is there life on other planets? That question has gone unanswered for millennia. The means to detect earth-sized planets around sunlike stars at earthlike distances is still somewhat in the future, but the methods and technologies necessary to find them continues to develop, bolstered by the seeming flood of new planets discovered every year. Perhaps in the not-too-distant future, a world will be found that may either harbor some form of life, or be suitable for habitation by earthlings.

(Conception of a terrestrial moon orbiting a gas giant)

(click here or on image for enlargement)

Methods:

There are several methods for detecting extrasolar planets in use today. The ones that will be discussed here are the method of transit observation, radial velocity measurements, astrometry, and photometry. Each one has its advantages and disadvantages, as will be briefly explained.

Transit Method:

The transit method looks for planets by, in essence, watching for eclipses. If a planet orbits a star in a plane that is edge-on or is very nearly edge-on from our perspective, looking from earth, we may see the light from the star become somewhat fainter as the planet crosses in front of the star. There are many reasons that a star could become fainter other than planet transit. A close orbiting binary companion could be alternating between eclipsing and being eclipsed, and the total amount of light seen from the pair would then grow and fade at regular intervals. The star could be suffering from a particularly hideous outbreak of starspots, such that they cover a significant amount of the surface and thus make the star on the whole noticably fainter. Some stars actually pulsate; the star itself periodically expands a bit, then contracts to its former state. When such a star is larger, it has a greater surface area emitting light, and the star appears brighter. Which reason turns out to be the case for a particular star dimming can be determined by other data and methods, which will be explained more below.

The reason that captures the attention of planet hunters is the possibility that the star was not eclipsed by a binary star, nor that it had starspots, nor that it pulsated, but it instead had a planet pass in front of it. Just as the moon darkens the sky during a solar eclipse, a planet passing in front of its parent star would appear to dim the star. The drop in apparent magnitude (brightness) would depend on the physical size of the planet and the geometry of the orbit - whether the planet crossed the middle of the star or just the edge. The duration of the transit ("eclipse") depends again on the geometry of the orbit, and also the orbital distance of the planet. A planet close to the star transits very quickly compared to a planet many times farther away. To confirm the presence of the planet requires observations of periodic dimming.

An advantage of this method is that one can observe a great many objects at once, as in this project. You simply observe a large group of stars over time and see which ones fade periodically. It does not necessarily require intensive study of one star at a time, although planetary system candidates can be reexamined in detail. A disadvantage is that the planets' orbits must be or nearly be edge-on as seen from Earth. Having no prior knowledge of a planetary system around a star, one must observe a great many stars before the probability of finding one with such an arrangement is decently high. The vast majority of stars may have planets but they wouldn't be discovered by this method because the system is not edge-on. Another drawback is the fact that in order to confirm a planet's existence it must be observed to transit more than once. Given an orbital period, the planet's distance from the star can be estimated. Since very prolonged observations are difficult to perform, it's likely that planets found will be extremely close in their stars, with orbital periods of only a few days.

Click here to see a further explanation of a planetary detection by transit, from the TEP Homepage (includes sample images).

Radial Velocity Method:

Ever noticed how the sound of that truck driving towards you seem to rise in pitch until it passes you (preferably to the side as opposed to over), and then goes down again? This is most commonly known as the Doppler effect, and the same thing happens with light. Light emitted from an object traveling towards you gets bunched up, or more specifically the wavelengths are effectively shortened. In visible light, shorter wavelengths correspond to colors towards the blue or violet end of the spectrum. An object moving away effectively stretches out the wavelengths, which in visible shows up as more towards the yellows, oranges, and reds. Light compressed by an approaching object is "blueshifted," and that of a receding object is said to be "redshifted."

The radial velocity method involves analyzing the light from a star to identify whether it is alternatingly blue- and redshifted. Stars hold planets in their orbits by gravity due to their large masses, but planets have masses too, and they pull on the star as well. The star and planet both orbit around the center of mass of the system (much closer to the star as it has much more mass). As the star moves in its orbit, the light from the star as seen by an outside observer is alternatingly blue- and redshifted. The amount of shifting of wavelength depends on the velocity with which the star moves in its orbit, which in turn depends on the mass of the planet(s) pulling on it. Stellar models and observations give standards for what the actual emitted wavelengths are, so one can determine by what amount the detected light is shifted either way.

This method, like the transit method, requires that the system not be viewed from directly overhead. While the "edge-on" idea is not as strict in this case, the closer to edge-on the system is, the easier it is to detect the shifting of the light. So far as current technology goes, it takes a large planet close to its parent star to have sufficient influence on the star to be measured by existing instruments. As instruments become more refined, it will become possible to detect smaller velocity variations, like those from earth-sized planets.

The first extrasolar planets discovered, around a millisecond pulsar, were found by a variation of this method. Pulsars are like lighthouses, in that they emit light (well, radio waves) in a single direction as they spin around rapidly. A detector on earth, like the radio telescope at Arecibo, Puerto Rico, picks up a signal at each turn. Pulsars can be timed to great precision, as their rotation changes only very slightly over long periods of time (and at a predictable rate). Any delay in the signal from a pulsar indicates the existence of some perturbing object - a binary star companion, a planet, etc. In the case of PSR B1257+12, it turned out to be planets. Due to the precision possible in timing the pulsar, the data resolved into three planets, all roughly earth-like as far as mass, and close in to the pulsar like the inner three planets of the solar system.

Astrometry Technique:

The astrometry technique is very similar to the radial velocity technique, but it differs in that it's better suited for "face-on" systems. The radial velocity technique cannot be used for face-on systems because the star would be moving across the field of view, and not towards or away from the observer, so no redshift nor blueshift would be detected. Astrometry is directed at detecting this lateral motion of the star, where measurements are taken for anglular motion rather than velocitiy. The amplitude of the star's motion would provide clues as to the companion's mass and orbital radius, and the period of the star around the apparent center of mass would be the same as the orbital period of the planet.

Photometry:

Photometry includes several methods of analyzing image data. For example, one might look at several different images of the same object to see changes between them. Photometry as a supplement to the transit method may involve cross checking the drops in brightness with color images to see if the color of the star appeared to change. Color changes can be caused by either a change in temperature or by a different object passing in front of the star - in which case the observed color of the star is a mix of the color of the star and the color of the object. The "true" color of the star can be deduced from observations and models for stars which give the intrinsic colors of those stars, making it easier to identify when the color change is due to the existence of another object.

Another photometric method, used for finding planets, involves attempting to block out the light from the parent star to isolate the light from the planet. Generally, the light from the star is so many billions of times that from the planet (which is really just light from the star but reflected by the planet), so the planet cannot be distinguished - it's lost in the light from the star. Photometry tries to even the field to help one see the planet directly. This can be aided by taking images in wavelengths other than those of visible light. Planets emit in the infrared portion of the spectrum and putout no visible light. Stars put out a vast amount of light in the visible bands, and a lot in infrared, but not nearly as much. If one looks in the infrared, the relative amount of starlight that must be blocked out is considerably less.

Another form of blocking out the star's light is the basis for a "nulling interferometer." This method involves using multiple images of the same star (and presumed system) taken from multiple telescopes (though it can be done with one). The idea is that with two "identcal" images, you can subtract one from the other thereby blocking out everything in the image that is more or less stationary in space (the central star, background features, etc), leaving only objects that may have moved (i.e., planets!). For more information on how this is applied, see the Terrestrial Planet Finder Book, technology section (Chapter 12).

Project Justification:

This project utilizes perhaps the best aspect of the transit method for planet detection, and that is that many stars can be observed at once. Indeed, they should be, since observations must be long-duration to increase chances of seeing multiple transits of the same planet. In observing the globular cluster 47 Tucanae, observations include several tens of thousands of stars. With that many, chances are much better that many will be edge-on (assuming a planetary system exists). Of these, there may be found several tens of new planets, if the planets of 47 Tuc have distributions of orbits and masses like those in the solar neighborhood.

Planetary system candidates can be studied in more detail later, by other methods like those described above. While transits may give clues as to size, radial velocity measurements lead to estimate for orbital distance. With estimates for those two quantities, one can make assumptions as to density and yield approximate masses of the planets. There can be photometry with emphasis on infrared wavelengths for the reasons stated above, or general multiwavelength spectroscopy to aid in determining composition (looking at objects in various wavelengths can reveal relative amounts of certain elements present).

The Hubble Space Telescope is especially well suited to carry out the quality observations required for this investigation. A telescope in orbit is less constrained than a surface telescope, for issues like sky conditions, ability to observe for only one part out of 24 hours, etc. HST can observe more constantly, cutting down the risk of losing a potential planet due to inability to observe one day, thus missing a transit.

This project can lead to further "mass planet searches," or broad searches that turn up a large number of planets with just a few observations. Although planets are not known to orbit solar like stars in globular clusters, it is entirely possible to significantly increase the number of known planets with the observations of 47 Tuc alone, if but a small percentage of the stars do have planets. This in turn will help in theoretical predictions of planetary system frequencies, and in establishing trends for certain planetary parameters.

        Related Upcoming Missions to Find extrasolar Planets:
            COROT
            Kepler
            Terrestrial Planet Finder (TPF)