Fortunately, a single planet’s signature is distinct from that of multiple planets. Solar systems follow gravity’s rules:
Let’s simplify the original question and consider only two planets orbiting the Sun: Earth and Jupiter. Earth would cause a velocity wobble in the Sun of only 0.22 mph (0.36 kilometer/hour) with a 1-year orbital period. Jupiter would cause a 27 mph (43 km/h) wobble, with a 12-year orbital period. State-of-the-art Doppler measurements have a precision of about 2.2 mph (3.6 km/h) we couldn’t even detect Earth. In our example, let’s arbitrarily increase Earth’s mass by a factor of 50. Its orbital period will still be 1 year, but the Sun’s velocity wobble will now be an easily detected 11 mph (18 km/h).
The velocity wobble for the 50 Earth-mass planet and the Jupiter-like planet are shown in the illustrations at the right. The star receives both tugs simultaneously, so what we observe are the added velocities. A 1-year sinusoidal wobble rides on top of a longer-period wave.
We could distinguish a nine-planet system from a single, more massive planet because each planet has a unique orbital period. With a single planet, there is only a single beat — a single wobble. In multiplanet systems, we see a different beat from each planet. If we lived on a planet around a nearby star, and if we had the sensitivity to detect all of the planets orbiting the Sun, then we would know the Sun hosts nine planets (we won’t get into a discussion of what constitutes a planet here).
But there’s a catch: To model this system, we need to observe at least one full orbit of each planet. Pluto takes 248 years to complete one orbit. Caterpillars move like bullet trains compared to the inch-per-hour Pluto-induced wobble of the Sun. So, mapping out the solar system’s architecture would span the career lifetimes of many astronomers and would require velocity-precision measurements technologically beyond our reach.
The final challenge to extreme velocity precision lies in the nature of stars themselves. If you look at movies of the Sun taken with the Solar and Heliospheric Observatory (SOHO), you’ll see the Sun’s surface convectively boils and flares. We measure the substantial velocities from the surface of our Sun with the same technique we use to find planets orbiting other stars. Stellar surface activity adds noise to our velocity measurements that will ultimately limit our accuracy.
Acoustic modes also cause the Sun to oscillate with periods of a few minutes and are also taken into account. I suspect stellar atmospheres will limit our ability to measure wobble velocities with precisions better than about 1.1 mph (1.8 km/h).
Science is a tag-team sport. By the time planet-hunters using the Doppler-wobble method reach extreme precisions and identify rocky planets at distances similar to Mercury or Venus, NASA will have a fleet of new space missions poised to take the next step. The Kepler mission (scheduled to launch in late 2008) will search distant stars and determine how often earthlike planets occur. The Space Interferometry Mission (scheduled to launch in the next decade) will find rocky planets in the habitable zones around our nearest neighboring stars, and other space missions will aim to image those planets. — DEBRA FISCHER, SAN FRANCISCO STATE UNIVERSITY