In the race to find the weirdest planet orbiting another star, we may have a front runner: GJ 667Cc, a super-Earth orbiting one star in a triple system that’s actually relatively closeby. And oh yeah: it just so happens to be in just the right spot to be potentially inhabitable!
Of course, I have some caveats, so don’t get too excited. But this is a weird and pretty cool one!
GJ 667 is a triple star system that’s right in our back yard as these things go: it’s only about 22 light years away, making it one of the closest star systems in the sky. It’s composed of two stars a bit smaller and cooler than the Sun which orbit each other closely, and a third, smaller star orbiting the pair about 35 billion km (20 billion miles) out. Stars in multiple systems get capital letters to distinguish them, so the two in the binary are GJ 667 A and B, and the third one is GJ 667C.
That third star is the interesting one. It’s a cool, red M dwarf with about a third the diameter of the Sun. Fainter, too: it only puts out about 1% of the light the Sun does. It’s been studied for years to look for planets around it, and while there have been some signs found, this new research is the first solid detection of planets that’s been published.
They used the Doppler method (sometimes called the Reflexive Velocity method): as planets orbit a star, their gravity tugs on it. We usually can’t see this motion directly, but a spectrum can reveal a Doppler shift, similar to the change in pitch you hear when a car or train goes by. If the spectrum has a high enough resolution, and the analysis very carefully done, there’s a lot you can tell by measuring it. You can get the planet’s mass, its period, and even the shape of its orbit.
In this case, the spectrum reveals GJ 667C may have four planets! Two very strong signals pop up with periods of 7 and 28 days, a third one at 75 days, and a possible trending shift in the spectrum that may point to a planet orbiting in a very roughly 20 year period.
It’s that second planet, GJ 667C with a 28 day orbit that’s so interesting. Its mass is at least 4.5 times that of the Earth, so it’s hefty. A 28 day orbit puts it pretty close to the parent star — about 7 million kilometers, or less than 5 million miles (Mercury is 57 million km from the Sun, by comparison). But remember, GJ 667C is a very dim bulb, so being that close means that the planet is actually right in the middle of the star’s habitable zone! The HZ is the distance where liquid water could exist on a planet — it depends on the size and temperature of a star, and also on the planet’s characteristics. A cloudy planet can hold heat better through the greenhouse effect, so it can be farther from the star and still be warm, for example.
A collaboration between space- and ground-based telescopes has added a new world to the growing list of exoplanets: Kepler-21b, a planet bigger and more massive than Earth. It’s far smaller than Jupiter, though, putting it firmly in the "small, rocky planet" category. Not that it’s Earth-like: it orbits its star in just under 3 days, making it hot enough to have pools of molten iron on its surface!
Now, I don’t generally write about every new alien planet discovered — with over a thousand of them and counting, it would be all I ever do! — but this one interested me. For one thing, it’s not all that much bigger than Earth; it’s about 1.6 times our diameter. The size was able to be found because the planet transits its star: it passes directly between the star and us, blocking the star’s light a wee bit. The amount of light blocked depends on the size of the planet itself, so by carefully measuring that dip in brightness the planet’s size can be determined.
And did I say a wee bit? I mean a really wee bit! Here is a graph showing the planet’s effect on the starlight:
The vertical axis is the amount of light we see from the star, and the horizontal axis is time. You can see how the light drops a bit when the planet blocks the star. But look at the scale! The planet blocks a mere 0.005% of the star’s light! That’s an incredibly sensitive detection, and incredibly difficult to detect. Stars have all sorts of ways of varying their light output, from sunspots to intrinsic pulsing. All those effects had to be removed from the observations to find this weak leftover signal.
But that’s the power of multiple observatories. The star was observed by the orbiting Kepler observatory, designed to look for such planets transiting their stars. It was followed up by the ground-based Mayall and WIYN telescopes at Arizona’s Kitt Peak National Observatory for confirmation, and in total the planet was watched for over 15 months to determine its characteristics.
Even better, these combined observations tell us the mass of the planet itself. As it circles its star every 2.8 days, its gravity pulls on the star, subtly changing the spectrum of the star’s light. The more mass a planet has, the more gravity, and so the more it pulls on the star, and the bigger the effect on the spectrum.
In this case, the planet has a mass of no more than 10 times that of Earth, and is probably less. Read More
Sometimes it pays to look over some older data and re-examine it. An exoplanet called 55 Cancri e was thought to have an orbit that was just 2.8 days long when it was discovered. However, two researchers looked over the data and realized they got a better fit if the orbit were actually only 0.73654 days — just under 18 hours! This meant it orbited its star far closer than previously thought as well.
And while that may be somewhat interesting, it’s the implications for the planet itself that make this orbital revision so cool. Or actually, hot. And dense.
Right. As usual, there’s a story to tell here…
The planet was discovered using the Doppler method: as it orbits its star, the gravity of the planet tugs on the star, causing a very small shift in the spectrum of starlight. The problem is getting enough observations to nail down the planet’s period; you can’t observe when it’s up during the day, and that cuts into the ability to get a good sampling of measurements. The discovery data gave a good fit at 2.8 days, so that’s what astronomers assumed was the orbital period.
But there were gaps in the data, and that can mask the true orbital period. When the data were examined more carefully, the 18 hour period was seen. But was it real?