About a month ago, astronomers announced they had found a new exoplanet---this one, orbiting in the habitable zone of the nearest star to Earth. Proxima b is exciting because it's nearby, and someday someone might send a spaceprobe to it. Plus, it has a mass close to Earth’s---making it more likely to be livable.
But Proxima b is *also *notable because scientists know its mass at all. Many of the 44 potentially habitable exoplanets have been found by Kepler-style transit searches, which watch and wait for planets to pass in front of stars and eclipse some of their light. But that only gives you a measure of a planet's radius. Proxima b popped out of a different technique, one that, for the most part, hasn’t been sensitive enough to see planets like Earth. And advances in that technique---including a new instrument called EXPRES---could improve detection enough for scientists to find and weigh lots of other Earth-mass planets.
That exoplanet-finding method, called a radial velocity search, works by detecting the seesaw of a star pulled around by its planet’s gravity. The back-and-forth movement in each orbit takes the star ever so slightly toward us, and then ever so slightly away from us. Rinse and repeat, at regular intervals. When the star moves in our direction, its light waves appear a little squished---bluer. When it moves away, the waves appear stretched---redder.
Transits tell us a planet’s radius. Radial velocities tell us a planet’s mass. But a planet’s overall density, and so its composition, only comes from their powers combined. If scientists only knew how wide you were and not how much you weighed, they wouldn’t know if you were gassy or rocky. The same is true of planets. So being able to detect Earth-sized planets with the radial velocity method will help scientists to figure out if they actually are Earth-like. That’s an express goal of EXPRES, and the Yale lab led by astronomer Debra Fischer that is developing it.
EXPRES is a spectrograph, a device that can measure the regular, repeating shifts from radial velocity changes. They split incoming light up by wavelength---like a prism, if prisms provided lots of data. The previous state-of-the-astronomical-art, an instrument called HARPS, was installed on a 3.6-meter telescope in Chile by the University of Geneva in Switzerland in 2003. But HARPS doesn’t comb the star’s spectrum finely enough to see the littlest, lice-like planets. And it doesn’t take stars’ natural noise into account enough.
“We’re used to the idea that the exoplanet research is this booming industry,” Fischer says. But she begs to differ. For the past five years, she says, “our ability to detect planets has absolutely flattened out.” Proxima b, a planet 1.3 times the mass of Earth, is the smallest-mass planet yet found with the radial velocity technique. But scientists only found it because Proxima is so close (cosmically speaking) to us, as well as very close to its low-mass star. Because of those proximities, the back-and-forth showed up stronger than it would have in a different system.
Fischer (and every other exoplanet scientist) wants to find more worlds like that. But current instruments, including HARPS, can only pick up velocities of about 1 meter per second---10 times higher than Earth’s 10 centimeter per second pull on the Sun. “The precision of our instruments isn’t good enough,” says Fischer. “That’s what EXPRES is setting out to try to change.”
The team has changed the physical technology---the hardware---to make EXPRES more precise. But the EXPRES team also innovates on the software side, using simulations to inform hardware development and subtracting out intractable noise.
Here’s the problem: The red- and blueshifts in starlight don’t come only from gentle planetary prods.
Stars are not calm beasts. Their atmospheres roil, boil, and send out huge plumes of particles and radiation. “It’s sort of like a fountain going off in every direction,” says Fischer. That movement---along with the stars’ wholesale motion---shifts the light. And scientists have to disentangle which shifts come from the fountain and which from the planetary tug-of-war.
Planets shift the whole spectrum---from low-energy light to high-energy---at once, by the same amount. Atmospheric activity, on the other hand, causes different shifts in different parts of a star’s spectrum.
Think about ultraviolet images of the Sun compared to infrared ones, says Fischer. In UV light, huge loops swing out from the solar atmosphere---they’re moving fast and shine much brighter than the Sun’s disk. Their velocity shifts show up strong. But switch to an IR picture of that same time period, and where the loops popped out, you’ll see only little surface spots. Because of their smallness and their slowness, they don’t really reveal themselves.
That difference has let the team sift the global from the local. “As soon as you tell me there’s a difference, you’ve opened the door a crack,” says Fischer. “Now, we get to kick in the door.”
EXPRES will ultra-finely split the light according to wavelength. And then before the team uses wavelength to calculate velocity, they find the “stellar jitter” and sift it out. And by “they,” they mean astrostatistician Jessi Cisewski, who developed the sort-and-separate algorithms.
The EXPRES team hopes to deliver the instrument to its permanent home---the Discovery Channel Telescope at Lowell Observatory in Flagstaff---in May of next year, and begin work by September 2017. They will drive it to Arizona in a U-Haul truck, with hopefully some thematic playlists blaring in the background.
The Discovery Channel Telescope is a good fit for EXPRES because it has active optics that autocorrect for distortions. It’s also a marsupial: It has a pouch for five different instruments and can switch between them in just 60 seconds. That way, even if there’s just a little bit of open time on the telescope, operators can switch to the EXPRES pouch at the end of someone else’s project, and only waste a minute. EXPRES will get data on the regular, which is key to seeing the periodic signals from planets.
They’ll use that time to do the 100 Earths Project, looking at a few hundred well-studied stars all over the sky to find 100 Earth-ish-mass planets, creating a catalog big enough to actually draw conclusions.
The EXPRES team is, of course, not the only group hoping to make it big with small-planet measurements. The closest comparable instrument will come from the same Geneva scientists who developed HARPS. This project---called ESPRESSO---will arrive at the Very Large Telescope in Chile around the same time EXPRES gets to Arizona. “The planet Proxima Centauri b, which was recently discovered with HARPS, is a foretaste of what will be possible with ESPRESSO,” says Francesco Pepe, who leads ESPRESSO’s evolution. He says the hard- and software will also let them look into the atmospheres of other worlds from the surface of our own, exposing “the ‘inner nature' of exoplanets.”
In a 2013 presentation, Pepe recalled a time 10 years before, when HARPS first put eyes on the sky and scientists thought they had butted up against technological and stellar limits. Radial-velocity searches had gotten as good as they could get, people thought (silly people). “Today, we know that reality is different, fortunately,” he said.
Now, they will just have to see who makes that reality reality first. Regardless, the instrument name will begin with an E.
