Searching For Habitable Worlds Beyond Our Solar System

06/09/2023
By Edwin L. Aguirre

On Valentine's Day 1990, as NASA’s Voyager 1 spacecraft was leaving our solar system, mission controllers commanded the unmanned probe to turn its camera toward Earth and take one last shot before embarking on its long interstellar journey. 

Voyager captured a portrait of Earth from four billion miles away, appearing like a tiny, dim point of light floating amid the vast emptiness of space. That view inspired the late astronomer Carl Sagan to publish his 1994 book, entitled “Pale Blue Dot,” which detailed his vision of humanity’s future in space. 

Since then, astronomers worldwide have used advanced ground-based telescopes and orbiting observatories like Kepler, TESS, Hubble and the James Webb Space Telescope to examine the sky for other potential “pale blue dots”—Earth-like planets, orbiting nearby sun-like stars, that are capable of harboring life. To date, more than 5,000 confirmed exoplanets have been found in nearly 4,000 stellar systems scattered across our cosmic neighborhood. Of these, more than 50 are considered to be Earth-like. 

Among the scientists involved in this field of exoplanet research is Asst. Research Prof. Christopher Mendillo of the university's Lowell Center for Space Science and Technology (LoCSST). Last fall, Mendillo, along with Physics Prof. and LoCSST Director Supriya Chakrabarti and the project’s research team, successfully launched a planet-seeking telescope to the edge of the atmosphere from a NASA facility in Fort Sumner, New Mexico.

Called PICTURE-C, which stands for Planetary Imaging Concept Testbed Using a Recoverable Experiment—Coronagraph, the 14-foot-long, 1,500-pound telescope was carried to the stratosphere by an unmanned, helium-filled balloon 400 feet wide and several stories high that was released from the Columbia Scientific Balloon Facility in Texas on Sept. 28. 

Mendillo says balloons are well-suited to search for planets outside our solar system. 

“We’ve used sounding rockets before, but balloons are an amazing platform to use for exoplanet research because of their relatively low cost, ability to lift heavy payloads and long observing duration,” he says. 

PICTURE-C was designed, built and tested by a team of student and faculty researchers and engineers at the LoCSST lab facility with support from a $5.6 million grant from NASA. The project’s goal is to detect debris disks, interplanetary dust and, possibly, exoplanets around nearby stars. 

NASA recently awarded Mendillo a five-year, $7 million grant to develop the next generation of UML’s high-flying telescope, which will be dubbed PICTURE-D (Planetary Imaging Coronagraph Testbed Using a Recoverable Experiment for Debris Disks). 

“The grant will help bring our research group to a new level.” -Prof. Christopher Mendillo

NASA has also named Mendillo a Nancy Grace Roman Technology Fellow, an award that gives early career researchers the opportunity to develop innovative technologies while honing the skills needed to lead wastrophysics flight instrumentation development projects. He will receive $500,000 in funding as part of the fellowship program.

“The grant will help bring our research group to a new level,” Mendillo says, adding that it gives LoCSST the opportunity to add facilities or equipment to its lab and possibly to hire personnel. 

To the Threshold of Space 

The data gleaned from the PICTURE-C mission will not only help find new planets, but it will also shed light on the evolution of our solar system, says Chakrabarti, who is PICTURE-C’s principal investigator. 

“Our balloon missions will enable us to gain a better understanding of the processes and dynamics that formed our own solar system,” he says. 

PICTURE-C spent 14 hours observing nearby stars at an altitude of 127,000 feet— roughly 3½ times higher than the typical cruising altitude of a passenger jetliner—to get above 99% of the Earth’s atmosphere. 

“Atmospheric turbulence distorts and blurs our image of the stars,” notes Chakrabarti. 

After successfully completing its observations, ground controllers sent a command to release PICTURE-C from the balloon. They then deployed a parachute to slow down the telescope and allow it to land gently on the desert floor for reuse in a future mission. 

The primary stellar targets selected for this mission were Vega (Alpha Lyrae), which is one of the brightest stars in the sky, and Epsilon Eridani. 

“Dust had been detected around Vega, so it’s thought to have a very big and bright debris disk close to the star. It’s never been imaged directly in visible light, never been resolved,” says Mendillo. “Epsilon Eridani, which is also fairly bright, is believed to have a Jupiter-sized exoplanet orbiting the star, but no one has seen the planet yet.” 

In the coming months, UML scientists will analyze the data that PICTURE-C captured and publish the results.

‘Like Looking for a Marble Next to a Lighthouse Beacon’ 

Last year’s mission was the second and final flight for PICTURE-C. The first, in 2019, was an engineering demonstration flight. 

“That was also a big success, including the recovery,” notes Chakrabarti. 

The latest mission further validated many key technologies developed at LoCSST that are essential for the first direct imaging of exoplanets from a balloon. 

PICTURE-C featured a specialized optical imaging system, called a “vector vortex coronagraph,” which was coupled to a telescope with a primary mirror 24 inches in diameter. The coronagraph was designed to “mask,” or greatly suppress, 99.99% of the direct light coming from the host star so that small, faint objects very close to the star— such as planets or interplanetary dust that otherwise would be hidden in the star’s bright glare—could be studied in great detail. 

“This is important, since we’re trying to see planets that are more than a billion times dimmer than their host star,” Chakrabarti explains. “It’s like looking for a marble next to a lighthouse beacon.” To obtain the highest image quality possible, the coronagraph used an onboard active optical pointing control system and adaptive optics designed and built by Mendillo and Postdoctoral Research Associate Kuravi Hewawasam ’20. It used a deformable mirror that could change its shape at high speed and in real time, according to the wavefront of the starlight that was coming in.

“This control system could optically stabilize the light beam coming from the telescope and keep the coronagraph centered on the target star to an accuracy of one milliarcsecond, or better, throughout the camera’s exposure,” says Mendillo. 

“A milliarcsecond is equivalent to resolving an object approximately 2 meters wide on the surface of the moon, which is about 385,000 kilometers away.” 

Mendillo notes that the pointing stability they achieved had never been accomplished before from any platform, especially hanging from a balloon. “It performed better than the Hubble Space Telescope and the James Webb Space Telescope,” he says. 

At this level of sensitivity and precision, maintaining the telescope’s ultrasharp focus and pointing are critical to the mission, since the telescope’s performance determines how many exoplanets the researchers are able to detect. 

According to Mendillo, PICTURE-C was able to use its deformable mirror to create the first-ever high-contrast coronagraph image produced by an observatory not attached to the surface of the Earth. 

“This was a huge step toward our ultimate goal of directly imaging and characterizing Earth-like exoplanets from a balloon,” he says.

PICTURE-D: The Next Generation 

For the PICTURE-D project, the researchers will use the same balloon platform and same telescope, but are making several major upgrades to the coronagraph to improve its performance, according to Mendillo. 

Currently, Mendillo says, they can only search one side of a star at a time. Adding a second deformable mirror will allow them to look around the entire star at the same time at extremely high contrast. 

“We’re also collaborating with our partners at Leiden Observatory in the Netherlands and NASA’s Ames Research Center in integrating a new type of coronagraph that will allow us to make polarization measurements of debris disks, as well as look for planets around binary star systems,” he says. 

If everything goes well, the team hopes the next mission will be the actual science flight, with the telescope fulfilling the project’s science objectives. 

Chakrabarti says the team’s approach to research is unique. 

“We’re a small group. Everybody does everything,” he says. “This is the only group in the entire world who has been flying instruments in near-space to validate all these technologies. That’s our contribution to the field.” 

In addition to Mendillo, Chakrabartiand Hewawasam, the team members include senior mechanical engineer Jason Martel, Ph.D. students Sunip Mukherjee and Thaddeus Potter, and Physics Assoc. Prof. Timothy Cook, who is the project’s co-investigator, along with other collaborators, including scientists from NASA’s Jet Propulsion Laboratory and Goddard Space Flight Center. 

“PICTURE-D will be the fourth iteration of the PICTURE family of missions, which began developing spaceflight exoplanet imaging technologies back in 2005,” says Mendillo. 

“These missions are steppingstones along the path to an experiment that will one day take the first picture of a new pale blue dot— an Earth-like exoplanet circling a distant star.”