Finding Earth-like exoplanets with the composition and ingredients for life as we know it is the Holy Grail of exoplanet hunting. Since the discovery of the first exoplanets in the 1990s, scientists have been pushing the boundaries of exoplanet hunting with new and exciting methods. One of these methods is direct imaging, in which the host star is carefully blocked out within the observing telescope to reveal the orbiting exoplanets that were originally hidden in the star’s immense glow.
Only about 1.5 percent of confirmed exoplanets were discovered using this method. One reason for this is atmospheric turbulence, which makes ground-based telescope observations difficult. However, a team of researchers has proposed improving this method to find an Earth-like exoplanet while mitigating this turbulence.
Here, Universe Today discusses these results, published in a recent study published in Nature Astronomy, which examines the use of a combination of ground-based telescopes with a space-based “starshadow.” We also share insights from the study’s lead author, Dr. Ahmed Mohamed Soliman, a scientist and technologist at NASA’s Jet Propulsion Laboratory, discussed the motivation behind the study, how their method compares to current direct imaging methods and upcoming missions, and what next steps will be taken to make this concept a reality. So what was the motivation behind this study?
“Many people think that only large space telescopes like the Nancy Grace Roman Space Telescope, the James Webb Space Telescope or the planned Habitable Worlds Observatory can search for life outside our solar system, but they don’t know what our NASA NIAC-funded study – Hybrid Observatory for Earth-like Exoplanets (HOEE) – can do,” says Dr. Soliman told Universe Today.
For the study, Dr. Soliman and his colleagues proposed the use of a hybrid ground-space observatory concept that includes a 99-meter (325-foot)-diameter orbiting star shadow and multiple 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 the Atacama Desert (Chile), while the TMT is located in Hawaii (USA). As the starshade blocks the star’s bright light and reveals the previously hidden exoplanets, the ground-based telescopes will work to find out whether the exoplanets are Earth-like.
Dr. Soliman tells Universe Today that the goal will be to identify dozens of Earth-sized exoplanets. He also points out that this concept will take only minutes to identify entire solar systems, including Earth-like exoplanets orbiting Sun-like stars, and that it will only take hours to identify potential biosignatures.
“In addition, as shown in our Nature Astronomy studies, the ELT’s advanced adaptive optics can correct for atmospheric turbulence, enabling clear imaging of habitable exoplanets and detection of potential life under moderate weather conditions,” says Dr. Soliman told Universe Today. “The planet should lie within the star’s habitable zone, where conditions allow the existence of oxygen and water. For a Sun-like star, this corresponds to about 1 astronomical unit (AU), the distance between the Earth and the Sun. For nearby stars, this corresponds to an angle of about 0.1 arcsecond. HOEE can observe at an angle of only 0.058 mas.” [milliarcseconds] from the star.”
As already mentioned, the direct imaging method involves the use of special instruments in telescopes to block the star’s glare and reveal the previously hidden exoplanets. This instrument, called a coronagraph, is an internal blocking method, while the proposed star umbrella serves as an external blocking method. There are countless ground-based telescopes that use coronagraphs to study exoplanets, including the Very Large Telescope and the Magellan Telescope in Chile, and the Subaru Telescope and the Gemini North Telescope in Hawaii.
Examples of currently active space-based telescopes that use 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 strong enough to directly detect real Earth-like planets in habitable zones. Existing ground-based telescopes also lack the required contrast and resolution. A hybrid system combining a space-based starshade with large ground-based telescopes would significantly improve starlight suppression and angular resolution and enable direct detection.” Earth-like exoplanets possible.”
The Nancy Grace Roman Space Telescope is currently scheduled to launch between September 2026 and May 2027 and will operate at the Sun-Earth Lagrange point L2, located on the opposite side of the lunar orbit from Earth. To illustrate, this is where JWST is currently located, as it offers an uninterrupted view of the night sky while being protected from the heat of the sun and having clear radio communications to send data back. In contrast, HST currently orbits the Earth every 45 minutes, its communications are blocked by our planet when it attempts to send data back to NASA, and its view of the sky is also blocked by our planet.
In addition to the upcoming Nancy Grace Roman Space Telescope, another planned space-based telescope has the potential to deliver groundbreaking exoplanet research: the Habitable Worlds Observatory (HWO), scheduled to launch 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, potentially using a coronagraph or starshade to support its direct imaging techniques. But how does the starshade concept proposed in this study compare to HWO?
“HWO will be more flexible in terms of targeting and monitoring cadence,” says Dr. Soliman told Universe Today. “HOEE, on the other hand, can observe about several times faster because it uses a ground telescope that is about six times larger than the HWO. HOEE achieves an angular resolution of [about] six times larger and allows detection of planets embedded in circumstellar dust produced by comets and asteroids in exoplanetary systems. HOEE can be a technological stepping stone and complement HWO or even accelerate the characterization of exoplanets before the launch of HWO.”
The journey to bring a space mission from concept to reality often takes years, often decades, of designs, tests, funding proposals, rejections, approvals, more tests, redesigns, and countless staff committees deciding its fate. These ventures are often funding-driven but also require a statement about the scientific value of the mission. For example, the United States National Academies created the Decadal Survey, which is a 10-year plan that sets out scientific goals for planetary science, astrophysics, earth sciences, and space physics.
The most recent decadal survey was the Astro2020 Decadal Survey, which established three key goals for NASA’s space exploration for 2030 and beyond. These include the identification of habitable exoplanets, the study of black holes and neutron stars, and the evolution of galaxies. The starshade concept proposed in this study falls under the habitable worlds target for the Astro2020 Decadal Survey. So what are the next steps to make this hybrid space-ground starshade approach a reality?
“The next question now is: Can we actually build it and launch it?” Dr. Soliman tells Universe Today. “The starshade must be 100 meters wide and very light so that rockets can launch it into space and move it from star to star. It sounds difficult, but exciting progress is already being made at NASA’s 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 chart a clear path for a true HOEE mission aimed at achieving the starshade “To find the very first Earth-like planet.” orbiting around 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 that’s why we do science!
As always, keep up the science and keep looking up!
