Twinkle, twinkle, wobbling star

Photos taken by NASA’s Voyager 2 spacecraft mesmerized Sally Dodson-Robinson as a child — from the striking closeups of Saturn’s colorful rings to icy-blue Neptune with the dark spot where a storm the size of Earth was swirling about.

“I would look at them and imagine going there and visiting and thinking how amazing that would be,” said Dodson-Robinson, associate professor of physics and astronomy at the University of Delaware.

When those photos were taken more than 30 years ago, the only planetary system we really had in mind was Earth and its neighbors.

“But astronomers were asking, ‘Why should we be special?’ and their predictions were completely right,” Dodson-Robinson said. “The first exoplanet was discovered when I was in high school, and in grad school, I got to see this revolution, from 30 planets found, to thousands that people now know about today. It’s been exciting to watch this revolution in astronomy and to be a part of it.”

Dodson-Robinson is now leading UD’s participation in an international space mission called “Twinkle” that aims to answer some of humanity’s greatest questions, including the age-old “Is anyone else out there?” It is the first space science mission of Blue Skies Space Ltd., which was co-founded by researchers at University College London in the United Kingdom to deliver independent satellites providing high-quality data to the global scientific community. The Twinkle mission involves scientists from 15 universities and counting, in North America, Europe and the Asia-Pacific region. This video of the Twinkle mission provides other insights.​

Dodson-Robinson’s involvement in Twinkle is being supported by UD’s Department of Physics and Astronomy and by a generous donation from the Miles Family Endowment.

Once launched into low-Earth orbit in 2024, the Twinkle satellite will capture new data on thousands of celestial targets, including exoplanets — the planets beyond our solar system orbiting distant sun-like stars. The satellite will carry a spectrometer that measures and records the light emitted, absorbed and reflected by objects in space, from stars and planets and moons, to comets and asteroids. This includes light we can see, and light of slightly longer wavelength in the infrared spectrum, which is invisible to us.

Scientists can detect exoplanets trillions of miles beyond our solar system by analyzing the light signatures of stars. Telescopes in space, such as NASA’s Transiting Exoplanet Survey Satellite (TESS), look for changes in the light emitted by a star. As exoplanets move along their orbit, they will block some of the light from their host star when they pass in front of the telescope’s line of sight, and the telescope will record that dip in brightness. ​

Dodson-Robinson wants to use Twinkle’s light-spectrum data to develop a complementary way to search for Earth-like exoplanets and involve UD students in the process. (Besides her complex research, Dodson-Robinson teaches a course called “Introduction to Astronomy.”)

Her focus is on a star’s radial velocity — the ever-so slight elliptical movements of a star in response to a planet’s gravitational tug. This wobbling, as if the star is using a hula hoop, affects the star’s light signature. A star moving toward us during that wobble has light of slightly shorter wavelength, which reads bluer in color, versus a star moving slightly away from us, which will have a longer wavelength and thus be redder.

“The surface of a star is a very active place – bubbles arise from convection, there are flares, areas of strong magnetic fields, the stalling of gases. If we’re looking at motion,” Dodson-Robinson said, “how do you separate out this ‘noise’ versus the gravitational pull of a planet? That’s a big challenge because the wobbling of stars caused by the tugs from small, Earth-like exoplanets are minuscule and can easily be masked by this noise.” ​

Twinkle’s mission is explained in this video provided by Blue Skies Space Ltd.

The hypothesis she’s working on is that at some wavelengths, or colors of light, you will get more “noise,” or up-and-down motions, reflecting natural activity on a star’s surface rather than an exoplanet’s tugging.

“My hope is that, using Twinkle’s data, we can monitor specific colors and by removing the noise, we can develop models of this stellar variability and find Earth 2.0 right here from the ground,” Dodson-Robinson said. “Many exoplanets have been found from the ground, but not Earth-like ones orbiting sun-like stars.”

Twinkle is expected to provide more than 70,000 hours — nearly eight years — of observational data once it launches in 2024. Robinson’s work will complement other Twinkle research on the atmospheres of exoplanets to determine their habitability.

Finding another Earth out there will take extensive searching and validation. How close is a prospective candidate to its sun-like star? Does it have an atmosphere rich in oxygen, an ozone layer for protection against ultraviolet radiation, liquid water on the surface, and so on? Such “checklists” for livability are very involved, Dodson-Robinson said, driving intense climate modeling efforts in the search for other planets that can sustain life. ​

But planets have always enthralled Dodson-Robinson. Recently, the American Astronomical Society invited her to write about her specialty — giant planets — for their monograph series. The Origins of Giant Planets, the first volume published in December 2021, serves as both an introduction for astronomy students and postdoctoral researchers, as well as a useful reference for senior professionals in the field. The society highlights the book in an interview now available on YouTube.

What is a giant planet? In our solar system, they include Jupiter, Saturn, Uranus and Neptune. Rather than having a large, rock body like Earth, these planets are huge balls of gas surrounding small, dense cores. You couldn’t really stand on them because their surfaces are not solid. Yet they have had an outsized role in controlling the growth, composition and orbits of their smaller, potentially habitable neighbors.

Research has suggested that as Neptune grew, it may have starved the solar system of a supply of solids that could have formed a super-Earth, and as Jupiter grew, it directed asteroids into the inner solar system, providing a water source for early Earth — and those are just a few examples, Dodson-Robinson said.

“It will be impossible to unravel the formation of any habitable world without characterizing its giant-planet neighbors,” she said. “While they may be gas balls, there are good reasons to include giant planets in the search for extraterrestrial life.” ​

​Sarah (Sally) Dodson-Robinson received her doctorate in astronomy and astrophysics from the University of California at Santa Cruz in 2008 and afterward took a Spitzer Postdoctoral Fellowship at the NASA Exoplanet Science Institute. An associate professor of physics and astronomy at UD, Dodson-Robinson won the American Astronomical Society’s Annie Jump Cannon Award for her contributions to the study of planet formation. She conducts numerical simulations of the chemical and dynamical evolution of planet-forming disks and participates in observational studies of debris disks. She also develops and tests methods for distinguishing exoplanet discoveries from stellar noise.​

​ Article by Tracey Bryant; Photos by Kathy F. Atkinson
​Published May 24, 2022​