How much is out there at the edge of our solar system that we have not yet discovered?
S: The short answer is—a lot. The Kuiper belt is probably littered with hundreds, if not thousands, of ice-dwarf planets like Pluto. NASA has explored all four terrestrial planets and all four giant planets. But the number of bodies we’d classify as planets in the solar system is probably closer to 9,000 than it is to nine, and we haven’t been to the most populous class of bodies at all—the ice-dwarf planets of the Kuiper belt. Even farther out, beyond the Kuiper belt, lies the Oort cloud, 1,000 times farther away. The Oort cloud consists of objects ejected from the region surrounding the giant planets during and after their formation. In the Oort cloud there may be large planets that were ejected from the solar system in the early days when Jupiter, Saturn, Uranus, and Neptune were muscling out their rivals.
Could there be objects as big as Jupiter or Saturn in the region beyond Pluto?
S: Not objects the size of Jupiter or Saturn because Jupiter and the other giant planets couldn’t have ejected objects that large, but there certainly could be a handful of Earth- and Mars-size objects. There could be hundreds of things the size of Pluto in the Oort cloud, and a number of objects the size of
the Earth or Mars.
How do we find them?
S: You have to look! Actually, anything in the Oort cloud is too faint to see with the technology we have today. For example, NASA’s Spitzer Space Telescope, which launched in August
, can see an object the size of Pluto located a few hundred astronomical units away [one astronomical unit is 96 million miles—the distance from the Earth to the Sun], and Spitzer can detect a planet the size of Earth out to about 1,000 astronomical units. However, the Oort cloud is between 10,000 and 100,000 astronomical units away. So an Oort cloud survey will have to be the project of a future generation.
Can studying Pluto and these other objects tell us anything about Earth’s formation and its history?
S: Yes, it can. The standard model for the formation of the Earth-moon system is that a huge, Mars-size object hit Earth and spun off material that coalesced in orbit to become the moon. The Pluto-Charon system is the only other place in our solar system where we believe this happened on a planetary scale. By going to a system like Pluto-Charon, we’ll better understand how the Earth-moon system formed.
After Pluto, where else in the solar system would you like to explore?
S: I would like to see a robotic return mission to Uranus and Neptune. I’d like to see further robotic exploration of the Kuiper belt, and I’d like particularly to see humans go back to the moon for a serious exploration and then on to asteroids and Mars.
What are the vulcanoids?
S: The vulcanoids are a putative population of asteroids that may circle the sun inside the orbit of Mercury, like a little Kuiper belt, if you will, on the inside of the solar system. Instead of being icy, however, they are expected to be rocky because it is so hot. Although no object on such an orbit has been detected just yet, there is good reason to expect that they might exist. For example, the surface of Mercury is severely battered, and many of the projectiles that hit it may have been vulcanoids. The question is whether there are any left or whether they’re all gone. We really don’t know because it’s a very, very difficult observational problem to detect these bodies, even with modern technology. The easiest way to look is from space, but that’s very expensive. Doing it from the ground is extremely hard because one has to look for faint points of light against the twilight sky. Our group has taken a middle-ground approach by using high-altitude aircraft that can fly up into the high upper stratosphere where the sky is much darker, making the vulcanoids easier to detect. We haven’t finished our search yet, so I can’t yet tell you if there are vulcanoids. Stay tuned!
You’ve done these vulcanoid surveys and studies of other asteroids, meteors, and comets from the backseat of a high-performance aircraft, like a WB-57 or a F/A-18 Hornet fighter jet. Is there really a scientific benefit to working out of an F-18, or is it just a lot of fun?
S: It’s both. I have to say that the grants that paid for the studies came through peer-review panels that throw out five of every six proposals. If this was purely so that I could have a good time in the backseat of a fighter, it wouldn’t survive for 10 minutes in a peer-review panel.
My colleague Daniel Durda and I discovered a way to do certain kinds of astronomy from a jet that you just cannot do from the ground. You can do these studies from space, but this is 1,000 times cheaper, and 10 times cheaper than using a big aircraft like those NASA typically has flown around. And for the particular niche we are exploring, the jets we use do the trick.
As a pilot, do you ever get the itch to take the controls?
S: Every flight. Sometimes we even get to. But the mission isn’t to let the astronomer fly; it’s to accomplish specific observing goals with the gear we bring along.