Two astronomers from the Lowell-Flagstaff Observatory are among the co-authors of an upcoming study in Monthly Notices of the Royal Astronomical Society It will announce the discovery of six exoplanets and 13 additional planet candidates.
However, Gerard van Bell and Kathryn Clark aren’t just contributing authors. They designed and built the instrument the study used for the follow-up observations needed to confirm the existence of exoplanets.
QWSSI
Van Bell and Clark’s latest contribution to the search for exoplanets is the six-channel Wavefront Sensing Interferometer, which has replaced the previous observational differential survey instrument.
Speckle imaging is a technique used to reduce the hazy effects of atmospheric turbulence in astrophotography. It involves taking many short exposures of a target object, very short, in 100ms or less, so that the atmosphere has minimal time to shift during the photo. These exposures are then processed to recreate an original high quality image.
Speckle interferometry, as performed by QWSSI and other similar tools, involves incorporating additional data from diffraction patterns generated by integrating speckle images into the reconstructed image of the target object, using Fourier analysis of noise cancellation and correct granulation.
Photographing stars through the atmosphere, Van Bell explained, “is a lot like trying to look at a quarter at the bottom of a swimming pool after someone has just jumped in”. “This QWSSI instrument helps us untangle this…it allows us to use the telescope at full resolution.”
QWSSI is installed on the 170-inch Lowell Discovery Telescope, the fifth largest telescope in the United States mainland, which is located southeast of Flagstaff at Happy Jack. It can simultaneously image at six wavelengths, four visible and two near-infrared, and also includes a wavefront sensor to provide additional data about incoming light that can be used in post-processing. Its predecessor, the DSSI, was only capable of imaging at two wavelengths.
“The design philosophy is optimized for quick and inexpensive construction,” Clark and Van Bell wrote in describing QWSSI in the Bulletin of the American Astronomical Society in 2021. 3D printing.”
According to Van Bell, the design work took about six to nine months, and was greatly facilitated with the use of off-the-shelf parts, for which they were able to build a virtual prototype using CAD files provided by the manufacturers. Assembly required another six months.
Van Bell commented on the build time: “We were able to put together a schedule and keep it, which is nothing short of amazing.”
Also of note is the QWSSI’s price drop of around $26,000. Van Bell noted that a key factor in keeping costs down was their decision to recycle CCD detectors from DSSI, which saved about $100,000. Because they have successfully demonstrated the capabilities of QWSSI, Van Bell said, they have received three additional grants to replace those detectors, add an additional set of detectors, and upgrade filters with purpose-built optics. The latter will cost $120,000, as opposed to the cost of off-the-shelf parts for the tool.
The assembled QWSSI fills an optics box about two square feet, six inches thick. Testing started in the summer of 2020.
“It worked pretty much right out of the box when we installed it on the telescope,” said Van Bell with pleasure.
Clark took a leading role in correctly assembling and harmonizing the tool, and he is also one of the primary users of QWSSI data, which requires a certain kind of mathematical expertise.
Van Bell is an astronomer at Lowell Observatory specializing in interferometry and planet detection. He is also the chief scientist for the Marine Optical Precision Interferometer. The asteroid 25155 Van Bell is named after him.
A former research assistant, Clarke earned her Ph.D. from Northern Arizona University with a thesis based on the development of QWSSI and is now a postdoctoral fellow at NASA’s Jet Propulsion Laboratory, where her research focuses on the characterization of planets in multiple star systems.
Kepler corrections
The Kepler space telescope, launched in 2009, is designed to detect exoplanets by measuring the dip in stellar brightness caused by planets crossing their parent stars. Observed 530,506 stars and discovered 2,662 planets over the course of nine years.
In 2013, two of the reaction wheels on the telescope failed, disrupting its aiming functions and reducing its light-gathering ability by more than an order of magnitude. The lower resolution of the data produced by this second phase of the Kepler mission required improved methods of analysis and additional follow-up observations to identify possible exoplanet signals within it.
In the TFAW survey, led by Daniel Del Ser of the Royal Academy of Sciences and Arts in Barcelona, Kepler curve data for light are processed by “a series of pixel correlation and degradation algorithms” called EVEREST 2.0, and then fed into a “wave-based narrative,” Del Ser and colleagues wrote in MNRAS paper: A shattering and noise reduction algorithm” called TFAW that “provides better photometric accuracy and characterization of planets than any shattering method applied to K2 light curves”. The results of these algorithms go through an additional audit process to weed out false positives.
To ensure accurate analysis of the Kepler data in these ways, the research team found it necessary to first update the background data of the target stars to exclude stellar companions. This data was provided in part by QWSSI and LDT, which provided between “thousand to several thousand speckle frames” for each star, according to the study. Van Bell described it as “doing a lot of cleanup”.
In addition to the LDT test in Flagstaff, the study made use of follow-up observations from the Pan-STARRS telescopes on Maui, the LAMOST telescope in northern China, and the SOAR telescope in Chile.
Planet fall
The second phase of the TFAW survey, documented in the MNRAS paper, analyzed more than 300,000 Kepler light curves and identified 27 candidate planets in 24 star systems. The team statistically confirmed six planets and rejected eight candidates as false positives. Confirmation of the existence of the remaining 13 candidates requires additional follow-up observations.
The authors summarized: “Our sample of validated and candidate planets consisted of three sub-Earths, seven terrestrial planets, four super-Earths, and four sub-Neptunes.” Most of these planets have an orbital period of three to ten days, which indicates that they are all within the habitable zones of their stars. Eight of the validated candidate planets have radii less than 1.5 times the radius of Earth, which the team proposes “indicates improved detection of smaller planets by combining TFAW-corrected light curves with TLS.”
Confirmed planets are:
- EPIC 210768568.01: 2.34 Earth masses, orbital period of 3.2 days, 965 light-years from Earth
- EPIC 247422570.01: 5.58 Earth masses, orbital period 5.9 days, 2181 light-years from Earth
- EPIC 246078343.01 & EPIC 246078343.02: 825 light-years from Earth. The inner planet has a size of 0.36 Earth masses and an orbital period of 0.8 Earth days, making it one of the very few planets discovered with a period shorter than one Earth day. The exoplanet has a mass of 1.83 Earths and an orbital period of 5.33 days.
- EPIC 246220667.01 & EPIC 246220667.02: 835 light-years from Earth. The inner planet has a size of 1.22 Earth masses and an orbital period of 4.4 days. The outer planet is 5.03 Earth masses and has an orbital period of 6.7 days.
Unconfirmed planet candidates for the study include EPIC 247560727.02, which is an exoplanet of a multi-planet system 2,220 light-years away, has a mass of 6.78 Earth masses, orbits its star in 8.4 days, and due to its density, could be a water world. .
Looking forward to tomorrow
In addition to providing data for the TFAW study, QWSSI has also been used to survey the immediate vicinity of the solar system to about 50 light-years away to determine how many stars in this region are binaries. Van Bell stated that they examined approximately 1,200 stars during this survey and found about 35 new companion stars that were too faint to be detected by earlier instruments. These new discoveries raised the stellar multiplicity rate by about 10%, which has important implications for exoplanet hunting, as binary stars are unlikely to hold a planet in a stable orbit.
“We can discover things that no one has seen before,” Van Bell said.
QWSSI is expected to continue work finding exoplanets and characterizing binary stars together with the Yale Exoplanet Laboratory’s EXPRES High Resolution Spectrometer, another LDT instrument. A version of QWSSI classified for spaceflight is also under development.
“It was so much fun,” said Van Bell. “We’ve made it happen.”
A preprint version of the MNRAS paper is available at arxiv.org/pdf/2210.10805.pdf