This column first appeared in The Tablet in April, 2009.
Diagram of how the shadow of a transiting planet or moonlet affects the brightness of the object it is transiting. Courtesy Hawaii Institute for Astronomy
Last week [April 2009], NASA’s Kepler space telescope sent back its first images of a star field in the constellation Cygnus. Launched in March , Kepler has been slowly positioning itself far from Earth’s bright clouds; unlike most spacecraft, it’s not a man-made moon orbiting the Earth, but a man-mad asteroid following the Earth around the Sun, with a “year” a few weeks longer than Earth’s. Now, from the darkness of its orbit, it has aimed its telescopes at the Milky Way to look for traces of Earth-like planets.
The plan is simple. For the next three years (six years, if all goes well) it will be watching the same star-filled region of the Milky Way, carefully measuring the brightness of every star in its field of view – some 100,000 of them – looking to see if any one of them periodically dims by a hundredth of a percent or less. Such a small, regular change would be just what one would expect if a small planet were crossing the face of that star. In this way planets as small as Earth might be detected.
Notice that we won’t actually be able to see those planets. We will only know they are there by the shadows they cast… a shadow extending hundreds of light years, from the star and planet to our telescope. And, of course, we will only find those systems that by chance are lined up just right so that the planets cross between us and their stars. But by tracing their shadows, we will know that such planets are there; and we’ll learn which of those 100,000 stars will then be worthy of further study.
While the Kepler mission is getting underway, a team of us at the Vatican’s telescope in Arizona this week [again, April 2009!] are looking for a similar sort of shadow: the trace of a tiny moonlet crossing the face of the dwarf planet Haumea. In this case, we already know the moonlet exists. It has been seen in telescopes more powerful than ours. What we are trying to learn is the nature of the dwarf planet it is orbiting.
Haumea is only slightly smaller than Pluto, one of a half-dozen known dwarf planets, smaller than planets but bigger than asteroids or comets. But unlike Pluto, Haumea appears to have a very odd shape. Its brightness varies strongly and rapidly, as if the light we see were being reflected off a pinwheeling Rugby ball, two thousand kilometers long and a thousand kilometers wide. But even with the largest telescopes we can’t actually see the shape of Haumea directly.
Instead, we’ll use its moon’s shadow to help us trace out its shape. Observers all over the world are carefully timing the slight dip in Haumea’s brightness seen as the moonlet crosses the dwarf. Each telescope, at slightly different locations on Earth, sees the moonlet from a slightly different angle, and so each observation traces a slightly different path across Haumea. How long the dip lasts, tells us how wide Haumea is along that particular path. Combining all the different timings should allow us to map out the overall shape of Haumea.
Using shadows to detect unseeable things hearkens back, in one way, to the “Allegory of the Cave” in Plato’s Republic, whichsuggested that what we see of reality is a mere shadow of the Truth. But I see a different analogy, to a different cave. It is in the absence of light from the star that we learn of the presence of the planet. It is in the absence of light from the dwarf planet that we learn its true size and shape. And it is in the absence in the Tomb that we learn of the Presence that fills all shapes, and enlightens all shadows.
Of course, the success of the Kepler mission is well known, with several thousand planets discovered so far. Alas, our attempts to see shadows on Haumea were less successful. It happens...