Hot Times for Cool Stars

By Mark Zastrow
BU News Service

Alien worlds may be even more alien than we thought.

Artist’s impression of sunset on the super-Earth world Gliese
The M dwarf Gliese 667 C hovers on the horizon with its two companion stars in this artist’s impression of a sunset on the potentially habitable planet Gliese 667 Cc. (ESO/L. Calçada)

For decades, astronomers’ visions of a habitable planet circling distant stars were like holding up a mirror to our own: it had to be rocky (not gaseous like Jupiter), at just the right distance from its star to support liquid water (not too hot, not too cold), and orbiting a friendly yellow star like our own.

But within the last two years, scientists analyzing data from NASA’s planet-hunting Kepler space telescope have overturned that last assumption. It seems we were blinded by our biases; it turns out that the stars that hold the most rocky planets with just the right temperature are not like the Sun. Instead, they are a long-overlooked class of stars, the most common in the Galaxy: the diminutive M dwarfs. Now, astronomers are paying attention to them. If these systems harbor alien life, we might be able to detect them in less than ten years.

Compared to the Sun, M dwarfs are puny, with half the mass and half the diameter at most. They’re cooler, with surface temperatures that max out at 6700º F, compared to the Sun’s 10,000 ºF.  They give off a faint red glow much dimmer than the Sun’s fiery yellow. To be warm enough to support liquid water, any planet orbiting such a star would have to be close. The star would loom large in the sky, causing tides so great, they would stop the planet’s rotation. One side would be locked into perpetual day, the other in eternal night.

Until recently, astronomers had no reason to think there would be many of these planets around M dwarfs. To be honest, M dwarfs were kind of an afterthought,” says Andrew Howard, who coauthored an August 2012 study of Kepler’s first batch of data when he was a postdoctoral fellow at the University of California–Berkeley. NASA had launched the space telescope in 2009 specifically to find habitable worlds around stars like the Sun. But when Howard and his team ran the numbers on Kepler’s first 1200 exoplanets, they found a surprise: there were many more Earth-like planets around small stars than around those that resembled our sun. They had overturned the very assumptions that begat the satellite’s conception.

These miniature M-dwarf systems filled with rocky planets were an entirely new species of solar system—and highly prized by planet hunters who began to refocus their efforts. “This is where it starts,” says Andrew West, an astronomer at Boston University who specializes in M dwarfs. “When this came out, I was just like, ‘Holy crap. That is awesome.’”

Confirmation soon followed. In January 2013, a Caltech team led by astronomer Jonathan Swift published another analysis of the Kepler data showing that planets seemed to be everywhere—on average, one for each of the 300 billion M dwarfs in the galaxy. The next month, Courtney Dressing and David Charbonneau of the Harvard–Smithsonian Center for Astrophysics announced they’d found an error in the Kepler database that had been overestimating the size of most M dwarfs. This was even better news because astronomers measure the sizes of Kepler’s planets relative to the size of their host stars. If the stars were actually smaller, then so were their planets, meaning a lot more of them are Earth-sized.

With these revised figures, Dressing and Charbonneau concluded there’s a 95% chance that an Earth-sized planet that could support water around an M dwarf is less than 17 light years away, a tiny distance on a galactic scale. If the Milky Way’s diameter of 100,000 light years were shrunk to the size of the United States, that planet would be just a stroll across Central Park.

Artist's impression of the Gliese 667C system
In this artist’s impression of a vista on Gliese 667Cd, the three stars of the triple system are visible, as well as the planet Gliese 667Ce, a crescent to the left of Gliese 667C.

There’s no consensus yet as to why M dwarfs have so many rocky planets. One leading theory is that since stars and their planets are born out of the same cloud of gas, a small star simply doesn’t have enough leftover material to make large gaseous planets such as Saturn and Jupiter.

Whatever the reason M dwarfs produce rocky planets, it’s a lucky break for astronomers. With the limits of today’s technology, it’s also much easier to find Earth-like planets around M dwarfs than around stars like the Sun. That’s because it’s hard to find planets just by looking—they tend to hide in the glare of their bright stars. Instead, astronomers search for signs of the planets’ influence on their host stars.

They use two main methods. The first is to detect the gravitational wobble of the star as its planet pulls on it ever so slightly. As the star rocks toward and away from us, its light shifts between bluer and redder colors due to the same effect that causes the sound waves of a train’s whistle to shift pitch as it rushes past us. It’s easier to detect this shift in a tiny M dwarf because it gets yanked around harder than a more massive star. Plus, any habitable planets produce an even larger wobble because they are so close to their M dwarfs.

The other method for finding planets is to monitor a star’s brightness, hoping for a planet to wander in front of it, blocking some of its light and temporarily dimming it each time it “transits” its host star. NASA’s Kepler mission exploited this to great effect, using it to discover thousands of likely exoplanets. This, too, is easier with a small M dwarf because the planet blocks out a greater fraction of the star’s light.

GJ1214b (Artist’s impression)
Transit observations revealed the planet GJ 1214b around its M dwarf host, shown here in an artist’s impression.

Astronomers salivate at the sight of transits for another reason: it gives them the opportunity to detect life. A planet that transits also disappears behind its host star on the other side of its orbit. This allows astronomers to measure the star’s light alone. Then, they can subtract the starlight from their previous measurements, isolating the effects of the planet and its atmosphere.

After NASA’s next-generation James Webb Space Telescope (JWST) launches in 2018, one of its projects will be using this technique in an attempt to find the chemical footprints of organic molecules in exoplanet atmospheres. According to Jonathan Lunine, a planetary scientist at Cornell and member of the JWST science team, it will focus almost entirely on M dwarfs to maximize their chances.

Despite these advantages, any claim that a planet around an M dwarf is truly “habitable” comes with caveats. One is that the stars’ strong magnetic fields twist and writhe, releasing enough UV radiation to sterilize the planet. Life on Earth copes with similar flares from our own Sun, which occasionally disrupt satellite communications, but they’re much weaker—“microscopic in comparison,” says Howard.

Another worry is that when a star is born from its nebulous nursery of gas and dust, all that coalescing material keeps the young stars hot well into middle age. In an M dwarf, this period lasts much longer than a sun-like star—so long, that it could boil away a young planet’s oceans. “You basically get a Venus scenario,” says Gibor Basri, an astronomer at UC–Berkeley and an early proponent of M dwarf studies.

If there are oceans on planets around M dwarfs in which life can spawn and flourish, there is one thing working in its favor: time. As the M dwarf ages its rotation slows down, weakening its magnetic field and reducing the amount of UV radiation. So even if James Webb doesn’t discover any life around M dwarfs in our epoch, West is optimistic that life will have a chance in the future. Unlike our Sun, which will run out of fuel, sputter itself away, and die within 4 billion years, M dwarfs are “the VW Beetles of the galaxy,” says West. They can last for up to trillions of years—thousands of times older than the current age of the universe. “It may just be we haven’t been around long enough to really see M dwarf habitability,” he says. “You could imagine that when the first round of M dwarfs’ [magnetic fields] really shut off, then maybe life sets up. … We might be at the wrong time.”

How Do Astronomers Find Exoplanets?

Artist's conception of exoplanet Kepler-16b. Image courtesy NASA/JPL-Caltech/R. Hurt.
Artist’s conception of exoplanet Kepler-16b. Image courtesy NASA/JPL-Caltech/R. Hurt.

By: Sara Knight
BU News Service

Last month astronomer Erik Petigura announced it was likely that our galaxy may have up to 40 billion Earth-like, habitable exoplanets swirling around their Sun-like stars. Petigura’s conclusion, which resulted from his analysis of Kepler satellite data, marks a huge milestone in the search for exoplanets – a field that has experienced a rapid expansion in the past decade. The ultimate goal of this extra-solar quest – to find planets with conditions able to support life as we know it – is only attainable if researchers can not only locate these other worlds, but discern their composition. Given that the nearest exoplanet is 4.37 light-years, or 26.22 trillion miles, away from Earth, it is no simple task. So how do researchers find and analyze alien worlds?

It comes down to tenacious observation and a lot of math. First, astronomers must set a satellite or telescope to record a given patch of space – for example, the Kepler satellite focuses on an astral window of about 100,000 stars. Then they wait for minute fluctuations in the amount of light a star gives off, which indicates a body, maybe a planet, passing in front of the star. Researchers confirm the light-blocking object as an exoplanet only after noting that the light fluctuates in a regular pattern, which can take years depending on the length of the planet’s orbit.

Once the light-blocker is verified as a planet the researchers ascertain its volume and mass. Luckily for them, finding the planet’s volume is relatively straight-forward – the amount the star’s light dims as the planet passes by denotes its size. Finding the planet’s mass is trickier: researchers must determine the strength of the gravitational attraction between the planet and star. The larger the attraction, the more massive the planet. The star itself is also “in orbit” around the center of mass between itself and the planet – it is just so massive that its orbit is more of a wobble than a proper ellipse. As the star wobbles, the frequency of its light changes in the visible spectrum – a phenomenon known as the Doppler effect. By observing these fluctuations, astronomers can figure out how much a star wobbles, and therefore the mass of its orbiting planet.

Artist's conception of exoplanets in the Milky Way. Image courtesy Wikimedia Commons.
Artist’s conception of exoplanets in the Milky Way. Image courtesy Wikimedia Commons.
After obtaining the figures for the planet’s volume and mass, finding the planet’s density is as simple as dividing the mass by the volume. Once astronomers figure out the density, they can extrapolate the sorts of materials that may make up the world. For example, an exoplanet with an extremely high density is probably composed of heavier materials, like rocks and metals.

In 2010, astronomers began using light frequency analysis not only to find the mass of the exoplanet, but also to infer its atmospheric makeup. While the planet passes between the star and their observation point, the chemicals in its atmosphere will give faint light signatures, which astronomers analyze using a chemical spectroscope. By noting which elements shift in the star’s chemical lineup when the planet passes by, they can infer the chemicals present in the exoplanet’s atmosphere.

So far, astronomers have found a huge variety of alien planets – some made mostly of metal and with 20 Earth-hour years, others made mostly of super-hot gas and that have silicate glass particles as rainfall. While the exoplanets with what we on Earth would consider extreme conditions are the coolest to read about, the ones that are able to sustain liquid water most interest astronomers. Those relatively ho-hum planets reside in the habitable zones of their stars – zones that astronomers believe life as we know it in our solar system may exist. And despite the report that there are probably 40 billion of them out there, we have only spotted twelve of them so far. But by remembering the scope of our search this humble figure seems a little less discouraging. After all, we can only focus on a small fraction of space at a time and stars are oriented randomly throughout the Universe – who knows what we’re missing?