The search for habitable planets in other star systems has progressed considerably in the past few decades. As of the writing of this article, astronomers have confirmed the existence of 5,989 planets in over 4,500 planetary systems, with over 15,000 candidates yet to be confirmed. At the same time, next-generation observatories like the James Webb Space Telescope (JWST) have made amazing breakthroughs in exoplanet characterization. Unfortunately, scientists are still not at the point where they can characterize smaller planets located closer to their suns, where Earth-like planets are likely to reside.
In other words, the search for Earth analogs (Earth 2.0) is ongoing and is dependent on the deployment of more powerful instruments, like the Nancy Grace Roman Space Telescope (RST). But what if the instruments themselves could benefit from a redesign? In a new study, a team led by Prof. Heidi Newberg from the Rensselaer Polytechnic Institute (RPI) recommended building a telescope with a triangular mirror rather than a traditional circular one. They argue that a telescope of this design could provide a clearer path to discovering habitable worlds.
Heidi Newberg is a professor of astrophysics with RPI’s Department of Physics, Applied Physics, and Astronomy in Troy, New York. She was joined by colleagues from RPI’s Department of Mechanical, Aerospace, and Nuclear Engineering, and the Laboratory for Exoplanets and Stellar Astrophysics at NASA’s Goddard Space Flight Center. The paper detailing their findings was published on September 1st in Frontiers in Astronomy and Space Sciences.
This artist’s concept shows the volatile red dwarf star TRAPPIST-1 and its four most closely orbiting planets, all of which have been observed by NASA’s James Webb Space Telescope. Webb has found no definitive signs of an atmosphere around any of these worlds yet. Credit: NASA/ESA, CSA/STScI/Joseph Olmsted (STScI)
Finding Earth 2.0
Because Earth is the only planet we know that supports life, scientists are forced to take the “low-hanging fruit” approach when searching for habitable planets. This means focusing on rocky planets that orbit stars similar in size and temperature to the Sun (G-type yellow dwarf stars). There are roughly 60 Sun-like stars within 30 light-years of Earth, some of which may have rocky planets warm enough to have liquid water on their surfaces. As Prof. Newberg explained, this represents a major challenge for conventional telescopes with circular mirrors:
Even in the best possible scenario, the star is a million times brighter than the planet; if the two objects are blurred together, there is no hope of detecting the planet. Optics theory says that the best resolution one can get in telescope images depends on the size of the telescope and the wavelength of the observed light. Planets with liquid water give off the most light at wavelengths around 10 microns (the width of a thin human hair and 20 times the typical wavelength of visible light). At this wavelength, a telescope needs to collect light over a distance of at least 20 meters to have enough resolution to separate the Earth from the sun at a distance of 30 light-years.
In addition, the telescope must be in space since atmospheric interference makes it much harder to detect light from distant sources. However, the largest space telescope yet deployed is the James Webb Space Telescope (JWST), which has a primary mirror measuring 6.5 meters (~21.3 feet) in diameter. Moreover, the JWST’s complicated design led to substantial delays and cost overruns and was extremely difficult to launch. These challenges have led astronomers to consider other approaches for observing distant objects.
This includes launching multiple, smaller telescopes that fly in formation and can collect more light than a single mirror. This light is combined in a technique known as interferometry to create a clearer image of the objects being studied. This also presents challenges, since the telescopes must maintain extremely accurate distances between them, which is impossible using current technology. Another proposal is to use smaller telescopes that observe objects in shorter wavelengths of light. However, this requires that light from a star be blocked out so instruments can obtain high-resolution images of any exoplanets orbiting it.
While modern telescopes rely on coronagraphs to block out a star’s obscuring light, they are not yet sophisticated enough to achieve the resolution needed. Yet another possibility is to deploy a spacecraft called a “starshade” to fly tens of thousands of kilometers in front of the space telescope to block out light from the stars under observation. “However, this plan requires that two spacecraft be launched (a telescope and a starshade),” said Prof. Newberg. “Furthermore, pointing the telescope at different stars would entail moving the starshade thousands of miles, using up prohibitively large quantities of fuel.”
Artist’s rendering of the Diffractive Interfero Coronagraph Exoplanet Resolver (DICER). Credit: NASA/Rensselaer Polytechnic Institute
Triangular Telescopes
As an alternative, Prof. Newberg and her team proposed a telescope about the same size as the JWST operating close to the same mid-to-far-infrared wavelength (10 microns). However, their telescope concept is equipped with a 20-meter (~65.6 ft) rectangular mirror rather than a circular mirror measuring 6.5 meters. With a mirror of this shape and size, said Prof. Newberg, astronomers will be able to separate the light of a star from any exoplanet orbiting it:
To find exoplanets at any position around a star, the mirror can be rotated so its long axis will sometimes align with the star and planet. We show that this design can, in principle, find half of all existing Earth-like planets orbiting sun-like stars within 30 light-years in less than three years. While our design will need further engineering and optimization before its capabilities are assured, there are no obvious requirements that need intense technological development, as is the case for other leading ideas.
Their concept design is modeled after the JWST and the Diffractive Interfero Coronagraph Exoplanet Resolver (DICER), another notional infrared telescope developed by Prof. Newberg and her colleagues from the Rensselaer Polytechnic Institute (image above). Like the proposed rectangular telescope, this concept relies on a novel design incorporating two primary mirrors measuring a few meters in diameter, flat diffraction gratings that are ten meters (32.8 feet), and a simple coronagraph.
These concepts present potential alternatives to launching many small spacecraft, or a telescope and a starshade, that require extremely precise formation flying. This ensures a measure of cost-effectiveness and reduces the potential for failure since it does not rely on multiple launches. Combining these advantages in a single observatory, Prof. Newberg concluded, could help drastically narrow the search for Earth-analogs in nearby star systems:
If there is about one Earth-like planet orbiting the average sun-like star, then we would find around 30 promising planets. Follow-up study of these planets could identify those with atmospheres that suggest the presence of life, for example, oxygen that was formed through photosynthesis. For the most promising candidate, we could dispatch a probe that would eventually beam back images of the planet’s surface. The rectangular telescope could provide a straightforward path towards identifying our sister planet: Earth 2.0.
Further Reading: Frontiers Science News, Frontiers in Astronomy and Space Science