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. This implies it’s denser than our home world. Why? Because at 1.6 times our diameter it has 4 times our volume (volume increases with the cube of the diameter, and 1.6 x 1.6 x 1.6 is about 4). If it had the same density as Earth that would mean it would have 4 times our mass. Since the mass is likely higher than that, it is probably denser.
That makes sense to me. The planet is orbiting its star at a distance of only about 6 million km — far closer than Mercury orbits the Sun! That makes the surface of Kepler21b hot, probably about 1900 K (roughly 1600°C or 3000° F). That kind of heat tends to boil away lighter stuff like water, leaving denser material behind (unless the planet is big enough to hold onto those lighter materials; Jupiter could, for example, but this planet is far smaller). So the planet being denser than Earth isn’t surprising.
Playing with the numbers a bit, I find the surface gravity of the planet is 4 times Earth’s, too. If you weigh 120 pounds on Earth, you’d weigh nearly a quarter ton on Kepler-21b. Not a great place to lose weight!
… on the other hand, with a surface temperature literally high enough to boil lead, you’d lose weight fast. But then, you’d be a puff of vapor. Probably not the best diet plan.
Not that you’d be heading over there anytime soon anyway. Kepler-21b is 350 light years away, or 3.5 quadrillion kilometers (2000 trillion miles) away. Getting there would be tough. I suggest something easier, like doing 10,000 push ups a day.
Anyway, this is an amazing detection; the planet is pretty small, very far away, and its parent star very luminous. These all combine to make this a tough world to detect, but that goes to show you: we’re getting really good at this sort of thing.
How long before we find another Earth this way? I’m guessing not very long. A few years at most. If they’re out there, they can’t hide forever.
Credits: ESO/L. Calçada; Steve Howell and the Kepler team