Interview with Ann Druyan and Steven Soter

The gravitational pull of an unseen planet causes a star to wobble. As the star moves toward an observer, the wavelength of the star’s light is squeezed and becomes more blue. As the star moves away from the observer, the wavelength is stretched and the light becomes more red. Credit: exoplanets.org

Kathleen Connell: Help us take a stab at understanding the extraordinary proliferation of planets beyond our solar system. What is the evidence, and why should we care?

Steven Soter: For a long time, there was a theoretical expectation that other stars have planets, that planets are common. But there was no observational evidence, because it’s extremely difficult to see planets of other stars. They’re relatively non-luminous, and they’re very close to their stars compared to the distance between stars, so they’re lost in the glare of their stars. If you were to look back at our solar system from the distance of a nearby star, you could not image any of the planets, even the largest ones, directly. But starting in 1995, some sensitive new techniques were developed that could detect planets indirectly, because, when a planet moves around its star, it actually tugs on the star by its gravity. The star makes a small orbital motion around the center of mass of the planet-star system. Since the star is luminous, you can detect that motion by the Doppler effect, because in part of its orbital motion the star is moving away from us, and the spectrum is red-shifted. In the other half of its motion it’s moving toward the Earth, and the spectrum is blue-shifted. You can detect that subtle motion in the spectrum of the star. From the period of that motion you have the period of the orbital motion of the planet which is causing it. And from the speed of that motion you can calculate a lower limit to the mass of the planet. You can also detect more than one planet around a given star, because each planet makes its own contribution to the complex motion of the star. That’s been done in a number of cases.

Artist’s conception of a gas giant planet orbiting a nearby star. Credit: NASA and G. Bacon (STScI)

Starting in 1995, this technique began to yield results. Now there’s something like 80 exoplanets-planets of other stars-which have been detected by this technique. And they’re being discovered at a rate of about one a month. It’s looking quite likely that the number of planets in the Milky Way could outnumber the stars. That’s a major discovery of our time.

Kathleen Connell: We’re using inference-we’ve not imaged these planets, correct?

Steven Soter: Correct. There’s one other kind of detection. In one or two percent of all systems of exoplanets the plane of the planetary orbit is such that the planet will move in front of the star as seen from the Earth. It will make a transit, and will diminish the light output of the star by some small fraction, because the planet is a shadow across the disk of that star. And that will be repeated every orbit. That kind of transit detection method has also been achieved in one case. Again, it’s indirect. You don’t see the planet itself. But there’s nothing else that could be responsible for these two kinds of observations. No one has any doubt that these are companion masses that are orbiting the star.

Our Milky Way galaxy is packed with 400 billion stars and perhaps even more planets. Credit: NASA

The masses are in the range of the giant planets of our own solar system and larger. We cannot yet detect small planets-Earth-size planets-by this gravitational method, because they don’t make a sufficient motion of the star. And we’ve not yet seen even Jupiter-mass planets that are at the distance of Jupiter from the Sun, because their orbital period is on the order of ten years or more, and these observations have only been going on for the last seven years. But as they continue, we will be able to extend the detection to more and more distant planets, which have longer orbital periods.

Kathleen Connell: So, Ann, we have a consensus in the scientific community that exoplanets are real. We’re talking about hundreds of billions?

Ann Druyan: We’re talking about more planets than stars. We’re talking about a galaxy and a universe of planets that far outnumber the stars. That was the great revelation to me of working with Steve on writing the show: to look up at the Milky Way and not only see a galaxy of four hundred billion stars, but to realize that hidden from our view is a galaxy of worlds that outnumber the stars.

We are just a few years away from launching an ultraviolet all-sky survey which could look directly at thousands of stars at once to find not these giant gas planets, but small, Earth-like planets. There’s no doubt in my mind that we will.