Guest writer astronomer Claude Plymate, chief wrangler of the McMath-Pierce Solar Telescope at Kitt Peak National Observatory, lets us in on some fascinating facts. Geek or non, you will enjoy Claude’s latest installment
How to Find a Planet
Imagine gazing at an extremely bright light bulb on a dark night with a 1 mm diameter ball bearing placed about 40 feet away from it. You can probably appreciate that the blinding light would make the ball bearing all but impossible to see. Now imagine looking at the light and metal BB from a distance of about 2000 miles away. Believe it or not, the tiny angle (0.77 arcseconds) between light bulb and BB can be easily resolved in modern telescopes but the dim little BB would be completely lost in the glare of the intense light source. This is what it is like to try and spot an Earth sized planet orbiting a one of our nearby stellar neighbors. Most stars, however, are much farther away making it that much more difficult to spot any potential orbiting planets. Hunting planets around other stars is hard work! It is quite a testimonial to human ingenuity that, despite the difficulty, as of this writing over 1700 planets have been discovered orbiting stars other than our Sun! To find these extrasolar planets, or “exoplanets”, astronomers have tried various techniques with varying levels of success. Only two techniques have so far proven to be very successful.
The Radial Velocity or “Wobble” Method
It’s not quite correct to say that the Earth orbits the Sun. Technically, both the Earth and Sun orbit about the center of mass of the of the Earth-Sun system. Picture a hanging mobile sculpture, the Sun on one side and the Earth on the other with a stick between them that the whole system is suspended from. Since the Sun is much bigger and heavier than our little Earth, the mobile is suspended from a point close to but not centered on the Sun. When you spin the mobile, the planet sweeps out a large arc while the large Sun moves just a little.
The same is true for orbits. Planets swing around in their orbits, all the while gravitationally tugging on their host stars. A planet’s star reacts to this tug by circling around the balance point between them just like with the spinning mobile. Since stars are so much more massive than their planets, the radii of their orbits are correspondingly smaller. The point about which the star orbits is normally well inside the star itself!
Viewed from above – face on to the orbital plane of the stellar system – an orbiting planet will cause its star to wobble ever so slightly, like a boat rolling over waves, as it drifts through the galaxy. Astronomers spent decades carefully measuring the positions of the closest stars in our celestial neighborhood. Try as they might, no deviations in any star’s path due to a planetary system was ever detected. Either the perturbations were too small to measure or there simply weren’t any planets out there tugging on the stars.
Then, starting in the late 1980’s, astronomers tried a different approach. Instead of looking for the side-to-side wobble imposed by a face-on orbital system, they decided to use the in-and-out (radial) motion induced by an edge-on planetary system. If our vantage point happens to be more-or-less in the orbital plane of a star’s planets, the star will appear to move towards us half the time and away the other half of the time. A star’s radial (in-and-out) motion might seem like a more difficult measurement to make compared to simply plotting its zig-zag motion across the sky, but believe it or not, up until very recently (see the Photometric Method below), this is how the vast majority of extrasolar planets have been found. At the time I’m writing this, the total of number of planets found with the radial velocity method is 531!
How can astronomers measure such small radial motions in stars that are many light-years away? Spectroscopy. You are likely familiar with the fact that astronomers have long used “red shifts” of spectral features in distant galaxies to measure their “Doppler” velocity. A close look at any star’s spectrum (it’s rainbow of colors) reveals a plethora of gaps or dark lines breaking up the continuum. These “spectral lines” are due to the various gasses that make up its atmosphere. Every type of atom and molecule absorbs light at numerous precisely known wavelengths (colors). The star’s spectrum is then the combination of all the spectra of the gasses that make it up. Stars can then be categorized by their “spectral signature” – sometimes likened to a stellar barcode.
The radial velocity of a star or galaxy causes its entire spectrum to shift towards the red (longer wavelengths) if moving away from us or towards the blue (shorter wavelengths) when coming nearer. (This is how it was determined that the Universe is expanding. Hubble – the guy not the telescope – found that the farther away a galaxy is, the greater its red shift velocity tends to be. His conclusion that the Universe much be expanding was one of the major discoveries of the 20th century.) The same phenomenon is experienced with sound waves whenever a fast moving vehicle zips past making that characteristic Eeeeeeeeooooooooo sound.
We can easily distinguish that tonal shift in sound from relatively slow moving vehicles because the speed of sound itself is relatively slow (~349 m/s). The speed of light, however, is MUCH faster (~300,000,000 m/s), about 860,000 times faster! The Doppler shift seen in light is exceedingly small for slow moving objects. Still, by the 1980’s spectrographs had reached the exquisite resolution and stability required to measure the slow back-and-forth velocities of stars do-si-doing with their planets – at least their BIG planets! Small planets like the Earth are still beyond our detection limit.
Early detections were of strange so called “Hot Jupiters”, that is big planets orbiting very close to their stars. These planetary systems didn’t much resemble our solar system with its small terrestrial planets in close and the big gas giants farther out. We’d always assumed our solar system is rather typical. Is this not the case or might this simply be a “selection effect?” The bigger the planet the larger its influence on its star. Likewise, the closer the planet is to its star, the larger the perturbations of its parent star will be. So, the radial velocity method is most sensitive to big, close in planets. It’s of little surprise then that that is what we see! The farther a planet is from its star, the longer its orbital period. Finding smaller planets that are farther from their star, therefore, takes more spectral resolution and more time. To succeed in this business, you have to be both obsessively precise and patient.
Once the period of a planet’s orbit and its star’s mass (found from its spectral class) are known, determining the lower limit of the planet’s mass becomes a trivially simple calculation. The calculated mass is only a lower limit because we don’t know if we are looking truly edge-on to the stars back-and-forth motion or at some skew angle. Any deviation from straight-on will diminish the motion along our line of sight. To absolutely measure the size of a planet relative to its star takes…
The Transit or Photometric Method
Occasionally, by happy coincidence, we happen to be precisely aligned along the axis of an exoplanet’s orbital plan such that we see the planet transit across the disk of its star. This creates a mini-eclipse which ever so slightly dims the light we receive from that star. By plotting how much the star is dimmed, the relative size of the planet compared to its star can be calculated. Such measurements are known as Photometry (“photo” meaning light plus “metry” meaning to measure, is the measurement of light).
The axes of planetary orbits around stars are randomly distributed. They can be tipped at any angle from face on (looking down from above or up from below) to edge on where the orbital axis cuts right across the star. A planet the size of Earth at its distance from our Sun makes a really small target. For the Earth to appear to cut across the Sun as viewed from far outside the solar system, the viewing angle would have to be within about +/- 0.3 degrees of our orbital axis. From a random position, the chances of this are only about 1 in 300. To have a good chance of catching some planets that transit across the disk of their stars, you need to observe a LOT of stars! Also, remember that the Earth takes a year to go around the Sun.
To catch a transit of Earth from our imaginary vantage point outside of the solar system, we’d have to watch for a year to see just one transit. To be sure that what we saw was a planetary transit and not some random even (Sun spot, a random star moving across the primary target star, equipment problem, etc.), we’d need to see at least a 3 transits. And in the case of the Earth, this would take a minimum of 3 years. If you see a star dim once, you can’t say what caused it. If you see it happen twice, it’s always possible that it was caused by two separate objects. Seen a third time, you can be confident you know that it’s something in a regular orbit and you know what that orbital period is. So, not only do you need to observe a lot of stars but you need to watch them for a long time – at least several years!
Enter Kepler. Kepler is a NASA spacecraft that was launched in March of 2009. Its mission is to continuously stare at one area of the sky between the constellations Cygnus and Lyra. The spacecraft continuously monitors around 100,000 objects in a roughly 10×10 degree field of view. Being in space, Kepler does not have to contend with the shimmering fluctuations and weather variability of our atmosphere. This gives it an undisturbed and exceedingly stable vantage point resulting in unsurpassed sensitivity to stellar brightness variations. The Kepler science team recently announced more than 1200 candidate planets have been seen transiting stars in its field of view! On top of that, 54 of these planets appear to be at the right distance from their stars to be in the “habital zone” where planetary temperatures can allow liquid water to exist! Assuming these all pan out, this represents a huge jump in the number of worlds we now know about. This first list of exoplants is only the tip of the Kepler’s iceberg based on its first year of observations. We can expect many more discoveries to trickle in as Kepler continues its mission over the next few years.
Although Kepler’s transit approach to exoplanet detection is proving to be remarkably prolific, the method can’t tell us all that we’d like to know about the planets its finding. For this, other methods will need to be employed. Eventually, we should have telescopes that will be able to capture the spectrum of the planets themselves. A spectrum can tell us what makes up the planetary atmospheres. I don’t know when it’ll happen but someday we’ll hit pay dirt when oxygen and other molecules formed by biology are found in the atmosphere of an exoplanet. That will be the day when we indisputably learn that the Earth is not alone and unique in this galaxy. We will finally know that we have kindred among the stars and that life thrives throughout the Universe.
(c) GoshGusPublishing (ascap) 2011