Taking the Twinkle Out of Starlight

Measurements of the planet's brightness at different wavelengths will tell us about the planet's temperature and chemical makeup and whether the system has the conditions to support life. Observations in the thermal infrared region of the spectrum from 3 to 10 µm will be particularly valuable, because many simple organic molecules like methane emit strongly there. Furthermore, we will exercise many of the observational techniques and new technologies required to eventually find and study Earth-like planets.

The big challenge here is to distinguish a planet's light from that of its parent star. In the visible range, where planets shine by reflecting starlight, contrast ratios between a planet and its star can be extremely large. For example, the contrast ratio between Jupiter and the Sun is on the order of 10¹⁰.

Younger giant planets, less than a billion years old or so, still retain much of the heat created by their coalescence out of the primeval matter from which their solar systems were formed, and radiate strongly in the thermal infrared. For such planets, the contrast ratio may be improved to a mere 10⁶--still an enormous challenge. But most planetary systems, like our own, are thought to be much older. They will have cooled and will no longer glow in the thermal infrared as they once did.

A further complication occurs when trying to capture an image of the stellar system at the telescope. Regions of the image close to the star, where its planets might be found, are swamped by a halo of starlight scattered by Earth's atmosphere. The halo adds photon noise orders of magnitude greater than the tiny planetary signal. To have any hope of finding the elusive planet, we must rely on adaptive optics to suppress the halo as much as possible.

We will also do ourselves a favor by imaging at a wavelength of around 5 µm, where the contrast between planet and star is the lowest. That is where the very low thermal background radiation coming from the MMT adaptive optics system provides a crucial advantage, by reducing the photon noise against which the planet must be seen.

What's more, the stellar halo can be suppressed still further through destructive interference, using a technique called nulling interferometry. In this procedure, the images from two telescopes are overlapped exactly and in such a way that at the location of the star, the crests in the light waves from one telescope fall on the troughs of the waves from the other, canceling each other out. Thus, a very dark spot is created where, before, the bright stellar image was found.

The principle of conservation of energy requires that the starlight not be destroyed, and indeed it appears at a second output of the nulling interferometer. It is removed, though, in the crucial region closest to the star where we would expect to look for planets. The critical geometry needed to fulfill this nulling condition pertains only over a tiny slice of the image, corresponding to the fundamental resolution limit of the combined telescope pair. Planetary images in adjacent regions will remain, therefore--now with greatly improved contrast.

In a groundbreaking experiment, we have begun tests at the MMT of a prototype nulling interferometer in combination with the adaptive optics system. Instead of using two separate telescopes, the interferometer divides light from the 6.5-meter aperture into two parts. Additional optics then recombine them to satisfy the nulling criterion. In one of our first results, light from the primary star was suppressed so that it appeared no brighter than the secondary star. During the measurement, the adaptive optics maintained a stable high-resolution image well corrected for atmospheric turbulence.

As we continue to develop this program, further improvements in our instrumentation will allow us to see fainter objects. The next major step will be the completion in 2005 of the Large Binocular Telescope, combining two 8.4-meter primary mirrors on a single mount, each equipped with its own adaptive secondary mirror. The corrected light from the two halves of the telescope will then be brought together in the center in a new nulling interferometer now being built.

Predictions of the instrument's sensitivity show that we can expect direct detection of several planets already known to exist--for instance, those around e Eridani, 47 Ursae Majoris, and u Andromedae. Many others are likely to be discovered for the first time because of the instrument's ability to explore a much greater region of space around each star than is possible with today's indirect detection methods.