Venus was Just the Beginning: The Science of Planetary Transits

Are you sick of reading about the transit of Venus this year? Yes? Me too. But the fact is that when astrophysical objects move between us and something else, like the convenient blaze of a star, there is an extraordinary amount that can be learned. I won?t go far into the delights of a venusian transit here, since so much has already been written about both the historical viewing and this year?s viewing (don?t stare directly at the Sun for goodness sake), except to say that in case you?re curious here?s a small listing of some Venus transits of the past, present, and future (thanks to NASA):

Transits of Venus:  1601-2400                             Date       Universal    Separation                                         Time     (Sun and Venus)                          1631 Dec 07     05:19         940"                         1639 Dec 04     18:25         522"                         1761 Jun 06     05:19         573"                         1769 Jun 03     22:25         608"                         1874 Dec 09     04:05         832"                         1882 Dec 06     17:06         634"                         2004 Jun 08     08:19         627"                         2012 Jun 06     01:28         553"                         2117 Dec 11     02:48         724"                         2125 Dec 08     16:01         733"                         2247 Jun 11     11:30         693"                         2255 Jun 09     04:36         492"                         2360 Dec 13     01:40         628"                         2368 Dec 10     14:43         835"

You?ll note a couple of things in this table. First, as widely promised, the upcoming transit on June 5th/6th is indeed the last one for us Earth-dwellers until December 11th 2117, so keep taking those antioxidants if you want to catch it again. Next is the list of visual Sun-Venus separations in the 3rd column, these are measured in seconds of arc (precisely 1/3600 th of a degree) and refer to the distance between the center of the solar disk and the closest point Venus gets to that center. None of them are zero (which would indicate a transit across the solar center), and that?s because neither Venus or the Earth orbit the Sun in exactly the same plane as its equator. In fact we?re relatively lucky, this June 2012 transit is among the closer ones that can occur. Now if you?re very astute, or just plain compulsive, you?ll also see that the recurrence interval of transits follows a pattern ? they?re always separated by either roughly 8 years, 121 years, or 105 years. So it seems good fortune is indeed with us, this is only the sixth transit since the invention of the telescope, and comes a mere 8 years after the last one.

As nifty as this is, the really exciting aspect of transits are coming today from the study of exoplanets. For example, NASA?s space telescope Kepler is designed to spot the minuscule eclipses of planets transiting stars that are thousands of light years from us. Using this technique it has unveiled more than 2,300 good candidate exoplanets ? helping us transform our understanding of the abundance of exoplanets in the Milky Way galaxy. These transits are the result of a chance geometric alignment that increases in probability the smaller the planetary orbit and the larger the star, but the real treasure comes from the details of every transit event ? the so-called ?lightcurves? that trace the brightness of a star as time goes by. This great animation (NASA, Kepler) shows the lightcurve (wait a few seconds) as a gas giant planet orbits its parent star and transits.

Lest this look too easy, note that the dips in light are shown greatly exaggerated. A Jupiter-sized planet and a Sun-like star will result in a roughly 1% transit dip, and an Earth-sized planet produces a mere 0.008% dip. Nonetheless, an instrument like Kepler can reach this level of precision if it detects enough transit events to construct a statistical ?wrap?,? adding all the data together. You?ll also note in the animation that there is a hint of the light reflected by the giant planet itself, and even a little dip as the planet is eclipsed by the star. Such extraordinarily fine detail can indeed be seen, here for example in Kepler?s measurement of the giant planet HAT-P-7.

The HAT-P-7 stacked light curve from Kepler (Credit: Borucki et al. 2009)

There?s an awful lot going on in these lightcurves. The rounded shape of the transits is a consequence of the variation in brightness across the stellar disk (dimmer towards the edges due to the foggy opaqueness of a star?s outer atmosphere). Their depth is a direct measure of the relative size of planet and star, and their width is governed by the path taken across the stellar disk (which, like Venus, is typically not across the equator), as well as that disk?s size, and the orbital speed. If there were planetary rings, or moons, these too might show up as fine details in the shape and timing of the lightcurves.

Getting a handle on the physical diameter of an exoplanet is a huge deal ? armed with this, and an estimate of its mass (from the use of spectroscopic data measuring its gravitational pull on the star), we can begin to evaluate the actual internal composition of another world. I discussed this in some detail for the case of Kepler-22b a while ago.

But this is the tip of the proverbial iceberg. Clever use of transits has now enabled astronomers to not only make the first atmospheric temperature maps of a number of giant worlds, and some not-quite-so-giant worlds, but to also detect some of their atmospheric constituents. During a transit the starlight shines through the partially transparent outer layers of a planet?s atmosphere. Atoms and molecules in the atmosphere can leave an imprint in the spectrum of that light, selectively plucking photons out at very specific wavelengths. The result? We have now seen compounds like sodium, oxygen, carbon, methane, and even water lurking in the high altitude regions of a handful of large exoplanets. Eventually this technique may yield the first measurements of the atmospheric contents of an Earth-sized planet, something that the James Webb Space Telescope might be able to do for a couple of nearby systems.

Before we reach that breakthrough though, there is another extremely clever use of transits that reveals some truly unexpected qualities of exoplanetary systems. It?s a wee bit technical, but the Rossiter-McLaughlin effect dates back to the 1920?s through its application to eclipsing binary stars. For planetary transits the idea goes like this: stars rotate, and so the light we gather from a star is a mix of photons that have come from the side of the star rotating away from us (which redshifts them to longer wavelengths), and photons coming from the side rotating towards us (blueshifted to shorter wavelengths). So any specific spectral feature is smeared out, or broadened, due to this ? we always see that mix of photons.

But if a transiting planet now sails across the disk of the star it will sometimes block more of the redshifted photons and sometimes more of the blueshifted photons ? depending on its path across the sky. The two slides here are from a class I teach at Columbia University, and summarize the impact of this varying blockage of light.

In a nutshell, if you measure the spectrum of the starlight simultaneously with a transit you will find it shifting redder and bluer with a very specific dependence on the actual geometry of the path the transiting planet takes. It can appear as if the velocity wobble of the star due to the planet?s gravity is itself somewhat askew.

The truly wonderful thing about this is that we now have a way to measure the orientation of the planet?s orbit in relation to the spin-axis of the star. When you consider that we cannot see anything except the light coming from the star as a tiny point on the sky, to be able to dissect the orbital architecture of another solar system to this level of detail is simply quite astonishing.

And what have we discovered? Well, many of these planets orbit in planes closely aligned with the equator of the stars, and in the same direction as the stellar spin, much like our own solar system. But some don?t; some crazy, ditzy, unexpected worlds actually orbit in the opposite direction altogether! In fact, it appears that as many as 20% of closely-orbiting, hot Jupiter-type planets are in retrograde orbits.

Some planets have flipped (Credit: ESO, A. C. Cameron)

This is a big deal. The consensus opinion is that all planets forming from the disks of gas and dust surrounding young stars should initially be orbiting in the same sense as the rotation of the entire system. This means that something funky is happening in certain cases to fling these worlds into orbits that basically ?flip? into retrograde states.

It?s a fascinating, and unexpected, twist to the nature of planetary orbital architectures, that reveals a great degree of dynamical instability and change in the history of many objects. So, if you?re lucky enough to catch the last transit of Venus in our lifetimes, remember that this is just the beginning ? transits can reveal planetary sizes, rings, moons, compositions, temperatures, atmospheric contents, orbital arrangements, and will likely bring us many more delights long before Venus slides across our Sun?s face again.

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