Tuesday, January 31, 2017

Exoplanets Obey Kepler

Many years ago, this young, budding astronomy geek was just a naïve 10-year-old kid, marveling at the wonders of spaceflight, living the history of Mercury, Gemini, and Apollo literally as it happened.

Even as we reached for the Moon, we stretched toward the stars. There were a few scientific dogmas that we learned, and we were taught that these truths were pretty much established as fact for all eternity.

First, in spite of the fact that we were reaching the Moon in about three days, other inter-planetary travel was expected to be much, much more difficult. Mars could take a year to reach. Other planets even much longer. And inter-stellar travel? Captain Kirk might be able to put the Enterprise into warp drive, and the Space Family Robinson could reach Alpha Centauri after a few years in suspended animation, but we shouldn’t expect it soon.

No, it would take hundreds or thousands of years to reach the closest stars given the technology available. Space is very large and very, very empty. That’s why they call it “space”.

The second fact we learned was that stars could never be imaged as a disk. We really don’t see the stars; we see the light emitting from the stars. There’s a difference. Remember that first rule about space being really big and really empty? Stars will never be anything more than a point of light.

And planets around other stars? (We now call them exoplanets.) Well, the Enterprise and the Jupiter 2 seemed to bump into them all the time. But their existence was only hypothesized.

Yeah, stars probably had planets orbiting around them — why should Sol be the only lucky one? But — remember the first rule about space being really big and really empty? — we would probably never see them. After all, the stars were too bright and the planets too small. Stars generated their own light, but planets only reflected back a tiny part of that light into space. Our lifetime could only hope for the possibility of planets, not the reality of them.

The first rule still holds; space is still really big and really empty. But the ability to see stars and planets beyond our Solar System’s influence has been greatly improved over the last few years. We can now "see" exoplanets, but most are observed indirectly; we can see them only as the brief dimming of a star if their orbit lines up exactly with the Earth. Consider watching a mosquito fly in front of a searchlight a hundred miles away. It’s like that.

But since 2008, we have been able to spot a few planets directly. New advances in things like adaptive optics and the ability to block out the disk of the star are just a couple of tools that can now be used to directly observe planets. Yes, they are real!

It’s easiest to spot exoplanets directly when three conditions are true:
  1.  The planet is very large in relation to its host star — dozens of times larger than Jupiter helps.
  2.  The planet orbits far enough from the host star that it doesn’t get washed out in the star’s light.
  3.  The plane of the orbit is close to a right angle to the plane of the Earth and the host star. Observing planets by transit is only possible when the planet is in the same plan. Observing directly is easiest when it’s in a 90-degree angle. It’s a three-dimensional geometry thing. Work it out for yourself.

Fortunately, all those factors have come together to give us one of the most remarkable astronomical movies ever filmed. Seven years in the making. But worth the time and effort. It’s a thing of beauty:

Check the original article to see the picture in motion.

There they are: four beautiful planets, just as hypothesized. Each orbit obeying Kepler’s laws of planetary motion exactly as they should. In every case, the square of the orbital period of the planet is proportional to the cube of the semi-major axis of its orbit. Beautiful.

It’s been a long time coming. I haven’t yet seen flying cars or hoverboards. My Roomba is the closest thing I have to Rosie the robot maid. And I’m still looking for my personal jet pack!

But, hey, I’ve seen a movie of a remote planetary system. My life is now complete.

No comments: