Very cool news: Three new planets orbiting other stars — exoplanets — have had their picture directly taken!
Seeing planets around other stars is hard. Planets are faint, and stars bright: From a distance, for example, the Sun is a billion times brighter than Jupiter. Most times, when we look at other stars, the planets are totally lost on the glare.
Sometimes, though, we can make things easier. Planets are warm, so looking in the infrared helps. Looking at younger planets helps, too, since they still glow with the leftover heat of their formation. Cooler, redder stars are dimmer, making planets around them easier to see as well. Finally, checking out nearby stars is a big plus, since that means we can more easily separate out the planet and its star (like looking at a distant car blurs the headlights into one glow, but when it’s closer you can separate them).
After all these years, only about a dozen planets have been directly photographed, and even then some are controversial; their ages aren’t well known, and that affects their measured mass. Some might be brown dwarfs, objects intermediate in mass between planets and stars.
But now we have three more exoplanets baby pictures! Here’s what we know about them.
FW Tau b
FW Tau is perhaps the most interesting of the three systems. The star is actually a binary, two stars orbiting each other. Both stars are cool red dwarfs, about a quarter of the mass of the Sun each, orbiting about 1.6 billion kilometers (one billion miles) apart, roughly the distance of Saturn to the Sun. The stars are a bit less than two million years old, and are 470 light years from Earth.
The planet was first noticed in 2001, but was not confirmed as a planet until recently. This was determined by what’s called proper motion: The stars’ physical motion through the galaxy can be seen over time, and the planet is clearly moving with them.
The planet, FW Tau b, orbits both stars at a staggering distance of 50 billion kilometers, over 300 times farther away from its stars than Earth is from the Sun. For comparison, Neptune is only 30 times Earth’s distance from the Sun, so this new system is ten times wider than our own. From that distance, the two stars combined would only be about as bright as the full Moon as seen from Earth.
The planet has a mass ten times that of Jupiter, and is still hot due to its age, probably around 1700° C (3000° F), so it’s not habitable by any means. But it exists, and we have the picture to prove it.
ROXs 42B b
ROXs 42B is also a binary star, with both stars smaller and cooler than the Sun. They’re a bit under seven million years old, and 390 light years from Earth. The planet is also about 11 times the mass of Jupiter, but is closer in to its stars, about 22 billion kilometers (14 billion miles) away from them. That’s still many times farther out than Neptune is from the Sun.
Like FW Tau b, it’s moving through space bound to those two stars, and its that motion which gave it away as a planet orbiting them. It’s also quite hot, about 1900°C (3400° F).
The mass is somewhat hard to determine for this object, and may be anywhere from 6 – 15 times the mass of Jupiter. At the lower end it’s definitely a planet, but at the higher end it’s more like a brown dwarf. Some of this depends on the distance of the star from Earth; if it’s farther away, the planet must be more massive to be as bright as it is. The authors of another study of ROXs 42B b (published just two weeks before the other one, thouhg made public in October) are a bit conservative in the planet mass, but it seems to me that it’s likely to be part of a known and nearby cluster of young stars, meaning the planet is on the lower end of the mass scale.
ROXs 12 b
ROXs 12 is also a cool red dwarf, and also about the same distance as ROXs 42B is from Earth, 390 light years. The planet is bigger, about 16 ± 4 times the mass of Jupiter. As it happens, anything bigger than about 13 times the mass of Jupiter is more in the brown dwarf category; with the uncertainty in the mass of this object it’s not completely clear if it’s a planet or not. But it’s close, and for now (until better measurements are made) I’ll grant it, given the caveat. Either way, it orbits the star at a distance of about 31 billion kilometers (20 billion miles).
Interestingly, both FW Tau b and ROXs 12 b may have a disk of material around them, meaning they are still actively in the process of forming. The observations show they both glow in the light of warm hydrogen — with very young planets, that’s generally indicative of a disk. If so, we’ve caught them in the act of gathering material, building themselves up even more. That’s pretty cool (even if it means ROXs 12 b may be pushed out of the planet category as it gains mass).
I’m fascinated by these direct images of planets. For now, given our technology, the ones we see this way are young, hot, and massive. But we’re getting better at this. As we make bigger telescopes equipped with better detectors, and use more sophisticated techniques in our observations, more planets will be found. I have no doubt that we’re getting closer to being able to physically see a planet like the Earth, orbiting some nearby star. It may yet be several years away, but that time is coming.
I wonder how it will be, to gaze upon the light of a planet like that? To know that from there, our own planet would look much the same?
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