The search for terrestrial exoplanets is heating up by finding them around cooler stars. Freimann Assistant Professor of Physics Justin Crepp was part of team that recently discovered the most Earth-like exoplanet to date, Kepler-186f, around an M-star, the most-stable, slowest-burning class of star in the universe.
“This one is special because the planet is 1.1 Earth radii,” Crepp said. “Kepler-186f is the most Earth-like planet that we have found yet. 1.1 Earth-radii is in the habitable zone — and the error-bar overlaps; it could be the size of the Earth, maybe a little more real estate than the Earth.”
The planet was detected as part of NASA’s Kepler Mission, a research project based on using the titular telescope to monitor some 150,000 stars hundreds of light-years away and to uncover potential exoplanets via the transit method, Crepp said.
Since detection relies on planets passing in front of the star and blocking some portion of its light- – that is, transiting – it turns out that it is relatively easier to pin down a planet transiting a smaller star because it blocks a greater fraction of the light, producing a larger signal for researchers, Crepp said. This is the case with the M-star Kepler-186f, a class of star comparatively tiny on the stellar scale, and this fact entails other exciting implications.
“The habitable zone is closer because the star is smaller and has a lower luminosity,” Crepp said. “So if you’re giving off less light, then the planets need to be closer to be in the warm/temperate region where they could have liquid water. Last time, we called this region the ‘Goldilocks zone,’ where it’s not too hot and not too cold. Well, we’re again looking for Goldilocks-like planets, and Kepler-186f is in the habitable zone. But it’s closer to the star, and this is important because it increases the probability of the transit.”
Crepp’s previous work uncovered larger exoplanets of super-Earth-sized radii orbiting stars around a still larger star in the Kepler-62 system. What makes this finding so incredible is the smaller size of Kepler-186f, which means that the composition is almost certainly rocky, he said.
“If I did the thought experiment where I made all the material completely gaseous, I have to heat it up to room temperature because it’s in the habitable zone,” Crepp said. “Well, it turns out that it’s not massive enough; if it’s all hydrogen and you heat it up it will just evaporate – it will just go away. You have to have so much mass, so how do you do that? You have to have really high density. So what’s that made out of? Rock. We suspect strongly that it’s high-density, that it’s rock. It turns out, unfortunately, that we cannot measure the mass, only the radius, for this particular planet.”
The type of star the exoplanet orbits is intriguing as well, Crepp said. The super-sized, self-destructing supernovae-to-be that are O-stars are at one extreme of the size and volatility spectrum, with our sun, a G-star, somewhat intermediate in both size and volatility.
“And the other extreme is an M-star, so small that it’s just barely fusing hydrogen,” Crepp said. “So it’s sitting there, just burning, burning very slowly, very stably. And the age of universe is 13 billion years. This star, Kepler-186, will burn its hydrogen for 56 billion years. So why should you care about that? Well, that gives you a lot of opportunity for life to develop.”
One of the more provocative realizations from the Kepler project is the sheer abundance of this phenomenon, Crepp said.
“Why is this profound? M-stars are 75 percent of all stars,” he said. “And we can calculate the occurrence rate of planets around these kinds of stars, and we find that it’s tens of percent – it’s not one percent or 0.1 percent; it’s tens of percent. One out of every five stars has a terrestrial planet in the habitable zone. This is a result that Kepler is telling us – we did not know this a few years ago.”
Not only are the chances of finding an exoplanet similar to Earth around any given M-star surprisingly high, but also the enormous number of these systems in the universe makes finding an incredibly Earth-like entity almost a foregone conclusion.
“There’s another thing that’s going on here,” Crepp said. “How many stars are in the Milky Way? 400 billion. So 300 billion of them are M-stars. Kepler is telling us one out of five – so 60 billion, to an order of magnitude – 60 billion stars with a terrestrial planet comparable in size to the Earth, in or near the habitable zone. 60 billion. And that’s just one galaxy. There are a trillion galaxies. ... I do this for a living, and I still don’t comprehend it.”
To further the search for these highly probable occurrences of exoplanets, Crepp is building the iLocater, a near-infrared Doppler spectrometer to be used in the ground-based Large Binocular Telescope in Arizona, which will work with NASA’s Transiting Exoplanet Survey Satellite (TESS) to gather data on nearby exoplanets.
“So here’s the problem: Kepler-186 is 500 light-years away,” Crepp said. “If it has life, even if it can communicate with us, it’s a really slow chess game. Even if you travel at the speed of light, it takes 500 years to say ‘pawn to E6.’ So, we’re building an instrument at Notre Dame, iLocater, and it’s going to find these planets around that type of star. It’s specifically designed for infrared wavelengths; it turns out that to see an M-star, you can’t just look up in the sky and say, ‘Oh, there’s a nice M-star, and there’s a nice M-star.’ Your eyes aren’t sensitive to the light they emit, near-infrared. So there’s a technology hurdle there; iLocater works in the near-infrared, and it’s going to find planets around M-stars that are ten light-years away instead of 500. And we know, thanks to Kepler, that they must be there.”
In addition to finding these Earth-like planets much closer to Earth, the iLocater will gather unprecedented data on the planet’s mass and atmosphere to complement the radius measurements from TESS.
“And here’s what’s really profound about iLocater: When a planet transits, some of the light goes through the atmosphere, so you can actually figure what it’s made out of because it will absorb some of that light,” he said. “iLocater’s not just going to tell us the mass of the planet – by the way, when you combine mass and radius, you get the density of the planet, and that tells you what it’s made out of – iLocater’s also going to tell us the composition of the planet’s atmosphere. It’s going to give us low-resolution spectra. iLocater is tuned to find planets in the habitable zone around the stars that Mother Nature likes to make the most and also the planets that Mother Nature likes to make the most.”
Although there’s still a lot of work to accomplish, Crepp said he plans to have the iLocater launched in 2017-2018.
“It’s a lot of fun,” he said. “I insist upon having fun at work. It’s basically my one rule.”
