We know of nearly 2000 alien worlds orbiting other suns... single planet systems, like the one depicted here, may be the exceptions. Credit: ESO/L. Calçada/Nick Risinger

May 2, 2014 Alien worlds, extraordinary and mundane

Sen—We live in a time of wonder. When I was a kid, one of the biggest unsolved questions in astronomy was simply stated: are there planets orbiting other stars?

We knew of nine planets then, all orbiting our own star, the Sun. Some of the biggest telescopes using the best methods of the time were put to task on the problem. Many claims of planetary detections were made, but none panned out.

Then in 1992, the first planets were discovered. They orbited a pulsar, the burned-out core of a supernova. These were small planets, and probably formed after the star had exploded. This made them as alien as we could imagine, and so a bit difficult to empathize with.

Three years later, the first bona fide planet was found orbiting a star like the Sun. Since then, nearly 2000 more have been found, with hundreds of candidates awaiting confirmation. In 20 years we went from knowing almost nothing about exoplanets to knowing that the galaxy is filled with them. They may actually outnumber the stars themselves!

See? A time of wonder. But it comes with a price.

The incoming flow of new planets is so rapid that it takes an extraordinary discovery to merit being newsworthy. An Earth-sized planet; or one orbiting its star in the “habitable zone”, where liquid water can exist on its surface; or one so close to its star that its atmosphere is evaporating.

That’s amazing, and perhaps a bit numbing. We can get jaded so quickly sometimes; even I do, and I love exoplanets science. I was reminded of this fact this week when the Exoplanets app on my iPad notified me of the discovery of a new planetary system. Four planets were found orbiting a nearby star, it said, and I smiled a bit. Multiple systems are good; they give us more insight on how planetary systems form and evolve.

I clicked the link to the journal paper describing the discovery, and, in keeping with that ironic theme, it was simultaneously mundane and fascinating. The star, HD 141399, is rather unremarkable in many ways. It’s a bit beefier than the Sun, with a mass 1.14 times as great. It’s about 50% more luminous than the Sun, and appears to be nearing the end of its life as a normal star; in a few tens or hundreds of millions of years it will become a red giant. But for now, it’s pretty much a humdrum star like so many others, living out its quiet life in a sedate backwater of the galaxy.

But it harbors a secret only recently revealed: It hosts four gas giant planets, ranging from about ½ to one-and-a-half times the mass of Jupiter. The inner three have very familiar-sounding orbits: 94, 202, and 1070 Earth days in length; those are not so different from the orbits of the inner planets of our own solar system. The big difference is that in our inner system the planets are small and rocky, while those orbiting HD 141399 are far, far larger.

It was when I read about the fourth planet, HD 141399e, that I was amazed. In mass it’s between Jupiter and Saturn, and has an orbit 740 million kilometers in radius—almost exactly the same as Jupiter’s! That’s actually a rare commodity in exoplanets; most planets we find are either more massive than Jupiter, or far closer to their star, or both. Those kinds of planets are easier to detect, so we’re biased toward finding them. That makes HD 141399e something of a catch.

And it was hard to find. Many planets are found nowadays if they transit their star, physically blocking a bit of light from the star. We see that as a tiny drop in the starlight, and that can be detected using relatively small telescopes. But these planets were found because as they orbit their star they tug on it, and the star makes a little circle every orbit as the planet makes a big one. As the star moves toward and away from Earth over time, there is a tiny Doppler shift in its light due to its changing velocity, and that can be detected, measured, dissected. It reveals the periods and masses of the planets doing the pulling. This takes a much larger telescope, and involves very accurate and painstaking measurements. These measurements are fiendishly difficult to do, and that’s why this method is harder to perform. 

HD 141399 system

The HD 141399 planet orbits (white) overlaid on our solar system (gray). The green area is the habitable zone, where liquid water can exist on a planet. Note how similar HD 141399e's orbit is to Jupiter's.

But even all this wasn’t what really stunned me when I read about this fourth planet. It was the orbital period: 3717 days, or 10 years (again, very close to Jupiter’s 12 year period). That means that even though the astronomers have been observing this star for a decade, they have only barely covered one orbit of the planet. And the amount it tugs on the star is very small; the star’s speed as it makes its little circle is a mere 8 meters/sec: about 30 km/hr. I can ride my bicycle that fast!

Detecting it at all was a phenomenal piece of work. And this planet (along with its sisters) will help us understand how planets form, how their orbits evolve over time, and give us insight into our own solar system.

And that is, in my eye, what makes this ordinary star so extraordinary. Remember, this is just another star in the galaxy; there are billions just like it that you’d never notice sweeping the sky with your binoculars. But, like so many others, when you plumb its depths you find hidden treasures, planets both familiar and alien. And we can now do this, routinely.

We live in a time of wonder. Don’t ever, ever forget that.