Daily Roundup of the 224th meeting of the American Astronomical Society
Monday, June 2, 2014, 5:37 PM - Over 1,000 astronomers, journalists, educators and enthusiasts have gathered in Boston this week for the 224th annual meeting of the American Astronomical Society. The first day of presentations is already starting off with studies of suns spots and solar flares, but perhaps some of the most interesting are giving us a glimpse at the amazing research being done in the field of extrasolar planets, or exoplanets.
1. Astronomers identify a new type of planet. Introducing, the Mega-Earth.
The term 'super-Earth' has been making the rounds in astronomical circles in recent years, as some newly-discovered planets have been found that are too small to be gas giants like Neptune, but too large to be Earth-like world. However, one such planet - named Kepler-10c - is so big and so heavy, that it's earned its place as the first of a brand new type of planet - the mega-Earth.
Kepler-10c was discovered in early 2011, by the Kepler Space Telescope, as the planet orbited a Sun-like star roughly 560 light-years away, in the constellation Draco. When it was first identified, astronomers found that Kepler-10c orbited around its star every 45 days, and it was about 2.2 times the size of Earth. Based on what scientists knew of planets up until now, one of this size was more likely to be a Neptune-like 'ice giant' planet - with a rocky core but a thick atmosphere of hydrogen and helium - than it was to be a completely rocky world like Earth. However, the only way to be sure about this is to find out the mass of the planet. So, astronomers recently pointed the HARPS-North instrument, on the Telescopio Nazionale Galileo (TNG) in the Canary Islands, at the Kepler-10 star system, to measure Kepler-10c's mass, and they found that it is around 17 times more massive than Earth - much larger than expected, and too large for it to be a Neptune-like planet. Instead, that makes Kepler-10c a rocky world, but much bigger than your standard 'super-Earth'.
"This is the Godzilla of Earths!" said Dimitar Sasselov, a researcher at the Harvard Smithsonian Center for Astrophysics (CfA), according to a press release.
Also, their findings showed that this planet is very old, roughly 11 billion years old, thus it formed only 3 billion years after the birth of the Universe. That was a surprise, since the heavier elements that go into making rocky worlds were scarcer in that early time.
"Finding Kepler-10c tells us that rocky planets could form much earlier than we thought. And if you can make rocks, you can make life," Sasselov said.
With this discovery, astronomers won't necessarily have to limit their search for Earth-like worlds to younger star systems, which means there's more potential for life-bearing worlds out in the universe.
2. Exoplanets come in three basic 'flavours'
With the discovery of Kepler-10c, astronomers have definitely found a sub-class of worlds that deserves more attention, but in general planets seem to fall into one of three categories. Here in our solar system, we have examples of two of them - the small, rocky planets of the inner solar system and the large gas giants of the outer solar system. However, when looking out into the galaxy, astronomers have been finding a third type in abundance - those bigger than Earth but smaller than Neptune - which amount to around three-quarters of all the planets the Kepler telescope has spotted so far. Looking for a reason why planets fall into these three categories, a team of astronomers looked to the 'metal' content of the stars that these exoplanets orbit around - that is, the abundance of elements heavier than hydrogen and helium that they can detect in the spectra of light being emitted by the star.
The results showed three distinct 'flavours' based on the size of the planet, with rocky worlds ranging up to around 1.7 times the size of Earth, gas giants starting at 3.9 times the size of Earth, and the third type - the 'gas dwarfs' - being in between. A gas dwarf can build up a very large rocky core before gathering a thick hydrogen and helium atmosphere around it, with the core potentially growing larger the further out the planet forms from the star (this could be what gave rise to Kepler-10c). For the metallicity of the stars, their results showed a definite tendency for lower-metallicity stars (like our Sun) to develop rocky worlds, with higher metallicity stars producing more gas dwarfs and those stars with about 50 per cent more metals than our Sun having more gas giants.
According to Lars A. Buchhave of the Harvard-Smithsonian Center for Astrophysics (CfA), who led the study: "It seems that there is a 'sweet spot' of metallicity to get Earth-size planets, and it's about the same as the Sun," he said. "That makes sense because at lower metallicities you have fewer of the building blocks for planets, and at higher metallicities you tend to make gas giants instead."
One detail of note for this study is that it is based on those planets astronomers have already found, which tend towards ones closer in to their star. The details of the 'flavours' may change as astronomers discover more planets that orbit further out.
3. Space weather from red dwarf stars may be lethal to life
While the previous two studies gave us a better idea of where we can look for rocky worlds that might harbour life, this last one throws a bit of a wet blanket over the idea. A survey of red dwarf stars, which represent over three quarters of the stars in our galaxy, has shown that the harsh solar winds and solar flares (collectively 'space weather') emitted by these stars might doom any chances for life to develop on planets inside their habitable zone - the region around them where liquid water could exist on a planet's surface.
"The space environment of close-in exoplanets is much more extreme than what the Earth faces," said Jeremy Drake, an astrophysicist with the CfA, said in a press release. "The ultimate consequence is that any planet potentially would have its atmosphere stripped over time."
Not only that, but habitable-zone planets around red dwarfs have a good chance of being tidally-locked, due to their proximity to the star. This means that one side of the planet is always facing towards the star, similar to how one side of the moon always faces the Earth. For the planet, this would form what's been called 'an eyeball Earth' - with a harsh desert 'pupil' directly in the middle of the star-facing side, surrounded by an 'iris' of possible habitability, and then the 'white of the eye' wrapping around the rest of the world, which would be too cold, dark and icy to sustain much in the way of life.
Study co-author Ofer Cohen gives an idea of the extreme environment in the press release: "If Earth were orbiting a red dwarf, then people in Boston would get to see the Northern Lights every night. On the other hand, we'd also be in constant darkness because of tidal locking, and blasted by hurricane-force winds because of the dayside-nightside temperature contrast. I don't think even hardy New Englanders want to face that kind of weather."
There may be hardier forms of life that develop on worlds like this, but according to the research of Drake, Cohen and their colleagues, even if the planet had a magnetic field similar to Earth's (which shields us from the effects of solar flares and the solar wind from our Sun), it wouldn't help much for a planet orbiting a red dwarf. According to the press release, the computer model showed that there were times when an Earth-like magnetic field could hold strong, but those times were far outweighed by periods when the field would be much weaker. That would expose any life on the planet to the harmful effects of space weather, and could even prevent life from developing there in the first place.