Finding Earth-like exoplanets with the composition and ingredients for life as we know it is the Holy Grail of exoplanet hunting. Since the first exoplanets were identified in the 1990s, scientists have pushed the boundaries of finding exoplanets through new and exciting methods. One of these methods is the direct imaging method, which involves carefully blocking out the host star within the observing telescope, thus revealing the orbiting exoplanets that were initially hiding within the star’s immense glare.
Only approximately 1.5 percent of confirmed exoplanets have been discovered using this method, with one reason being atmospheric turbulence making ground-based telescopic observations difficult. However, a team of researchers have proposed enhancing this method with the goal of finding an Earth-like exoplanet while mitigating these turbulence effects.
Here, Universe Today discusses these findings that were published in a recent study in Nature Astronomy that explores using a combination of ground-based telescopes with a space-based “starshade”. We also share insight from the study’s lead author, Dr. Ahmed Mohamed Soliman, who is a Scientist and Technologist at the NASA Jet Propulsion Laboratory, regarding the motivation behind the study, how their method compares to current direct imaging methods and upcoming missions, and next steps in making this concept a reality. So, what was the motivation behind this study?
“Many people think only large space telescopes like Nancy Grace Roman Space Telescope, James Webb Space Telescope, or the proposed Habitable Worlds Observatory can search for life beyond our solar system, but they aren’t aware of what our NASA NIAC funded study – Hybrid Observatory for Earth-like Exoplanets (HOEE) can do,” Dr. Soliman tells Universe Today.
For the study, Dr. Soliman and his colleagues propose using a hybrid ground-space observatory concept involving an orbiting starshade measuring 99 meters (325 feet) in diameter and several powerful ground-based telescopes. These telescopes include the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT), with the ELT and GMT located in Atacama Desert (Chile) while the TMT is located in Hawaii (USA). With the starshade blocking the star’s glare, revealing the previously hidden exoplanets, the ground-based telescopes will work to identify whether the exoplanets are Earth-like.
Dr. Soliman tells Universe Today the goal will be to identify dozens of Earth-sized exoplanets. He also notes this concept will need only minutes to identify entire solar systems, including Earth-like exoplanets orbiting Sun-like stars, along with taking only hours to identify potential biosignatures.
“Further, as shown in our Nature Astronomy studies, advanced adaptive optics on the ELT can correct atmospheric turbulence, allowing clear habitable exoplanet imaging and potential life detection under moderate weather conditions,” Dr. Soliman tells Universe Today. “The planet should lie within the star’s habitable zone, where conditions allow oxygen and water to exist. For a Sun-like star, this corresponds to about 1 astronomical unit (AU), the Earth–Sun distance. For nearby stars, this corresponds to an angle of about 0.1 arcseconds. HOEE can observe at an angle as small as 0.058 mas [milliarcseconds] from the star.”
As noted, the direct imaging method involves using specialized instruments within telescopes to block the star’s glare, revealing the exoplanets that were previously hidden. This instrument, called a coronagraph, is an internal blocking method, while the proposed starshade serves as an external blocking method. There is a myriad of ground-based telescopes that use coronagraphs to study exoplanets, including the Very Large Telescope and Magellan Telescope in Chile, along with the Subaru Telescope and Gemini North telescope in Hawaii.
Examples of currently active space-based telescopes using coronagraphs to study exoplanets include NASA’s James Webb Space Telescope (JWST) and Hubble Space Telescope (HST), while the European Space Agency’s Solar and Heliospheric Observatory and India’s Aditya-L1 telescope use coronagraphs to study our Sun. But how does this hybrid concept compare to current direct imaging methods?
Dr. Soliman tells Universe Today, “Current space telescopes such as the James Webb Space Telescope and the soon-to-fly Nancy Grace Roman Space Telescope use internal coronagraphs for direct imaging, but their contrast is not deep enough to directly detect true Earth-like planets in habitable zones. Existing ground-based telescopes also lack the required contrast and resolution. A hybrid system that combines a space-based starshade with large ground telescopes would greatly improve starlight suppression and angular resolution, making direct detection of Earth-like exoplanets possible.”
The Nancy Grace Roman Space Telescope is currently scheduled to launch between September 2026 and May 2027 and is slated to operate at the Sun-Earth L2 Lagrange point, which is located on the opposite side of the Moon’s orbit from the Earth. For context, this is where JWST is currently located, as it provides uninterrupted views of the night sky while keeping free of the Sun’s heat and has clear radio communications to send back data. in contrast, HST currently orbits the Earth every 45 minutes, its communications are blocked by our planet when trying to send data back to NASA, and its view of the sky is blocked by our planet, too.
Along with the upcoming Nancy Grace Roman Space Telescope, another planned space-based telescope holds potential to deliver groundbreaking exoplanet science, which is the Habitable Worlds Observatory (HWO) and is slated to launch sometime in the late 2030s or early 2040s. HWO’s primary goal is to directly image and identify at least 25 Earth-like exoplanets and search for biosignatures while potentially using a coronagraph or starshade to assist its direct imaging techniques. But how does this proposed starshade concept in this study compare to HWO?
“HWO will be more flexible in terms of targeting and observing cadence,” Dr. Soliman tells Universe Today. “HOEE, on the other hand, can observe about many times faster because it uses a ground telescope roughly six times larger than the HWO. HOEE achieves an angular resolution of [about] six times greater, enabling the detection of planets embedded within circumstellar dust generated by comets and asteroids in exoplanetary systems. HOEE can be a technological steppingstone and complement HWO or even accelerate exoplanet characterization before HWO launches.”
The road to getting a space mission from a concept to reality frequently takes years, often decades, of designs, tests, funding proposals, rejections, approvals, more tests, re-designs, and countless personnel panels deciding the fate. These ventures are often funding-driven but also require explanation regarding the scientific value of the mission. For example, the United State National Academies created the decadal survey, which is a 10-year plan outlining scientific objectives for planetary science, astrophysics, Earth science, and space physics.
The most recent decadal survey was the Astro2020 Decadal Survey, which established three primary objectives for NASA’s space science for 2030 and beyond. These include identifying habitable exoplanets, studying black holes and neutron stars, and the evolution of galaxies. The starshade concept proposed in this study falls under the habitable worlds objective for the Astro2020 Decadal Survey. Therefore, what are the next steps in making this hybrid space–ground starshade approach a reality?
“The next question now is: can we actually build and launch it?” Dr. Soliman tells Universe Today. “The starshade needs to be 100 meters wide and very lightweight, so rockets can carry it into space and move it from star to star. It sounds hard, but exciting progress is already happening at NASA Jet Propulsion Laboratory, NASA Goddard Space Flight Center, and NASA Ames Research Center through NASA’s Starshade and NIAC programs. The Keck Institute for Space Studies has brought together top scientists and engineers to map out a clear path toward a real HOEE mission, one designed to find the very first Earth-like planet orbiting a Sun-like star.”
How will this starshade concept help identify Earth-like exoplanets in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!