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?
[UPDATE (January 15, 2011): Mea culpa. It's getting hard to keep up with all the exoplanets being found now, and a few folks have let me know that the planet CoRoT 7b, while bigger than Kepler-10b, is also likely solid and not a gas giant. In fact, they're pretty similar in size and distance from their respective stars! So this planet is not the first one of its kind to be found, though still very cool and exciting. My apologies for this, and next time I write about planets I'll make sure to go through the database first!]
Astronomers have just announced the discovery of the first planet orbiting another star that is unequivocally not a gas giant: it must be a very dense, rocky-metallic object not much bigger than the Earth!
The planet, discovered by the orbiting Kepler telescope, is called Kepler-10b. The star (Kepler 10) is roughly the same mass and temperature as the Sun, and is located over 500 light years away.
The planet was detected because it passes directly between us and the star as it orbits. When it does that, it makes a mini-eclipse, blocking a bit of light from the star. By knowing how big the star is and how much light is blocked, the size of the planet can be measured (the bigger the planet, the more light is blocked). In this case, Kepler-10b is only about 1.4 times the diameter of the Earth, making it the smallest exoplanet ever found!
However, there’s more. The planet’s gravity tugs on the star as it orbits, so as the planet makes a big circle around the star, the star makes a little circle in response (I like to use the analogy of a father dancing with his small daughter; as he swings her around she makes a big circle around him and he makes a little circle, because he’s much more massive than she is). As the star moves slightly toward and away from us we can measure the change in velocity using the Doppler shift, and that in turn tells us the mass of the planet. It turns out Kepler-10b is a lot more massive than the Earth, tipping the scales at 4.6 times the Earth’s mass.
So it’s not terribly earth-like; if you stood on its surface you’d weigh almost 2.5 times what you do now!
Today, astronomers announced that they have found a new exoplanet, a planet orbiting another star. Nearly 500 exoplanets have been found in the past 15 years, so what’s the big deal, you may ask?
The big deal is that this planet and star are from another galaxy!
[Artist's impression of the alien planet; click to extragalaticate.]
There is a whole lot of coolness going on here, so strap in.
OK, first, this planet is in our own Milky Way galaxy. The star, called HIP 13044, is about 2000 light years away, well inside our galaxy. So how do we know it’s from a different galaxy? All the stars in our galaxy orbit the galactic center, like planets orbit around a star. But many years ago, astronomers noticed that many stars in the sky have the same sort of motion as they orbit, as if they all belong to streams of stars, flowing like water in a river. Many such streams exist, and eventually astronomers figured out that these were the leftover remnants of entire small galaxies that had collided with, been torn apart, and basically eaten by our Milky Way.
Astronomers have announced the discovery of a planet with about three times the Earth’s mass orbiting the nearby red dwarf star Gliese 581. That in itself is cool news; a planet like that is very hard to detect.
But the amazing thing is that the planet’s distance from the star puts it in the Goldilocks Zone: the region where liquid water could exist on its surface!
Artist’s drawing (from 2007, before this announcement) of the planetary system of Gliese 581. Credit: ESO
First, a few things: 1) Gliese 581 is a dinky, cool red dwarf about 20 light years away. That’s pretty close as stars go; only a handful are closer. Bear in mind it’s still 200 trillion kilometers (120 trillion miles) away, and that’s still a bit of a drive.
2) The planet is one of six now known to orbit the star [that link goes to a PDF of the journal paper]. Apparently, all the planets have neat, circular orbits, so the system seems to be stable. This new planet takes 37 days to orbit the star once, and orbits at a distance about 1/6 the distance of the Earth from the Sun. As far as we know, it’s the fourth planet from its star.
3) The planets have all been found by the Doppler method: as they orbit the star, they tug on it. This causes a shift in the wavelength of emitted light from the star. The mass of the planet, its distance from the star, and the shape of the orbit all determine how the light shifts, which is how astronomers found those properties of the new planet.
OK, so that’s what we know. Now let me be clear here about stuff we can be fairly sure about.
Astronomers have confirmed that an object in an image from 2008 — thought at the time to possibly be a direct image of a planet orbiting another star — is in fact a planet.
I’ll explain in a sec, but I want people to understand that this discovery is being touted as the first direct image of a planet around another star. It isn’t. Nor is it the first direct image of a planet orbiting a sun-like star. What this is is the first direct image of a planet orbiting a sun-like star taken using a ground-based telescope. While that may sound overly picky, it’s actually a significant achievement, and worth noting.
First, the planet picture:
This image, taken in 2008 by the Gemini North telescope in Hawaii, shows the star 1RXS J160929.1-210524 (I’ll call it 1RXS 1609) in the center, and the planet (1RXS 1609b) indicated by the red circle. As I wrote about this in 2008:
This is extremely cool news: astronomers have, for the first time, directly seen an exoplanet orbiting its star from one side to the other!
This makes me happy scientifically, of course, but also for personal reasons. Let me tell you a story. Two, in fact…
1) [Story the First] Beta Pic: the star, the planet, the disk
The star in question is Beta Pictoris (or just Beta Pic to its friends), a very young star — it’s only a few million years old, compared to the Sun’s advanced age of 4.56 billion — with about twice the Sun’s mass and 9 times its brightness. As stars go, Beta Pix is pretty close, just 63 light years away, and is easily bright enough to be seen with the unaided eye from the southern hemisphere.
In the above picture, taken using one of the European Southern Observatory’s ginormous 8.2 meter units of the Very Large Telescope, Beta Pic is represented by the dot in the center. The star is so bright its light swamps everything around it,
so the star itself has been blocked by a piece of metal inside the camera that took the shot (that’s the reason for the dark circle in the center of the picture) so the astronomers subtracted the image of a nearby star to minimize the effects of the light from Beta Pic itself; this is a common technique I’ve used myself in Hubble images*. This allows us to see much fainter stuff near the star.
This picture is actually a composite of three separate observations. The outer part with the blue fuzzy stuff was observed in 1996, and I’ll get back to that in a sec. The good stuff is in the center: two images of the planet, called Beta Pic b, are superposed in the picture; it was observed in 2003 (left blob), then again in late 2009 (right blob). Observations taken just months before in 2008 and 2009 observation didn’t show the blob at all — it must have been too close to the star to be seen clearly — indicating this really is a planet orbiting the star, and not just some background object like a star or galaxy. In other words, astronomers have captured the motion of the planet as it physically moved from one side of the star to the other!
600 light years away, in the constellation of Auriga, there is a star in some ways similar to our Sun. It’s a shade hotter (by about 800° C), more massive, and older. Oddly, it appears to be laced with heavy elements: more oxygen, aluminum, and so on, than might be expected. A puzzle.
Then, last year, it was discovered that this star had a planet orbiting it. A project called WASP – Wide Area Search for Planets, a UK telescope system that searches for exoplanets — noticed that the star underwent periodic dips in its light. This indicates that a planet circles the star, and when the planet gets between the star and us, it blocks a tiny fraction of the starlight.
The planet is a weirdo, for many reasons… but it won’t be weird for too much longer. That’s because the star is eating it.
OK, first, the planet. Called WASP 12b, it was instantly pegged as an oddball. The orbit is only 1.1 days long! Compare that to our own 365 day orbit, or even Mercury’s 88 days to circle the Sun. This incredibly short orbital period means this planet is practically touching the surface of its star as it sweeps around at over 220 km/sec (130 miles/sec)! That also means it must be very hot; models indicate that the temperature at its cloud tops would be in excess of 2200°C (4000° F).