Astronomers envision a ‘High-Definition Hubble’ to look for life beyond Earth

Over the past two decades, NASA’s Hubble Space Telescope and other powerful observatories have collectively made extraordinary breakthroughs in our understanding of the universe: from black holes, to dark energy, to extrasolar planets, and cosmic evolution.

A direct, to-scale, comparison between the primary mirrors of the Hubble Space Telescope, James Webb Space Telescope, and the proposed High Definition Space Telescope (HDST). In this concept, the HDST primary is composed of 36 1.7 meter segments. Smaller segments could also be used. An 11 meter class aperture could be made from 54 1.3 meters segments.

Despite these breathtaking advances, humanity’s most compelling questions remain unanswered: Are we alone in the universe? Or, are other inhabited Earth-like worlds common in our galaxy? What’s more, how did life emerge from a chaotic cosmic beginning?

The Association of Universities for Research in Astronomy (AURA), based in Washington, D.C., spearheaded the study of space-based options for ultraviolet (UV) and optical astronomy in the era following the James Webb Space Telescope’s mission (planned for launch in 2018). 

The AURA report describes the scientific and technological case for building a “super-Hubble” space telescope that would view the universe with five times greater sharpness than Hubble can achieve, and as much as 100 times more sensitivity than Hubble to extraordinarily faint starlight.

These powerful capabilities would allow the observatory, called the High-Definition Space Telescope (HDST), to look for signs of life on an estimated several dozen Earth-like planets in our stellar neighborhood. It could provide the first observational evidence for life beyond Earth.

Though the report does not address a specific design for the HDST, its mirror would have to be at least 12 meters (39 feet) across to conduct a robust survey of nearby habitable planets. This would be accomplished by combining up to 54 mirror segments together to form a giant aperture. The construction of the Webb telescope’s 18-mirror mosaic provides an important engineering pathway to demonstrating proof-of-concept for this type of space observatory architecture.

The HDST would be located at the Sun-Earth Lagrange 2 point, a gravitationally stable “parking lot” in space located 1 million miles from Earth. The telescope would have a suite of instruments: cameras, spectrographs, and a coronagraph for blocking out a star’s blinding glare so that any dim, accompanying planets can be directly imaged. The construction would be modular so that astronauts or robots could swap out instruments and other subsystems. As with Hubble, this would ensure an operational lifetime spanning decades.

The motive for the HDST is driven in part by the discoveries of NASA’s prolific planet hunter, the Kepler space observatory. Kepler’s discovery of over 1,000 confirmed exoplanets provides a statistical database that predict Earth-like worlds should be common in our galaxy, and hence nearby to us and within observational reach of the HDST.

A 12-meter-diameter space telescope outfitted with a coronagraph could look for planets around an estimated 600 stars within 100 light-years of Earth. The Kepler statistics predict that 10 percent of nearby stars would host Earth-sized planets within the habitable zones of their stars, where temperatures are optimum for life, as we know it.

A simulated image of a solar system twin as seen with the proposed High Definition Space Telescope (HDST). The star and its planetary system are shown as they would be seen from a distance of 45 light years. The image here shows the expected data that HDST would produce in a 40-hour exposure in three filters (blue, green, and red). Three planets in this simulated twin solar system - Venus, Earth, and Jupiter - are readily detected. The Earth’s blue color is clearly detected. The color of Venus is distorted slightly becuase the planet is not seen in the reddest image. The image is based on a state-of-the-art design for a high-performance coronagraph (that blocks out starlight) that is compatible for use with a segmented aperture space telescope.

The HDST would spectroscopically characterize the atmospheres of these planets. The abundance of water vapor, oxygen, methane, and other organic compounds in the atmosphere could be evidence of an active biosphere on the surface of a planet.

Looking far beyond our local stellar neighborhood, the HDST would search for the origins of the chemistry of life in an evolving universe. The super-telescope’s UV sensitivity would be used to map the distribution of hot gases far outside the perimeter of galaxies. This would show the structure of the so-called “cosmic web” that galaxies are embedded inside, and how chemically enriched gases flow in and out of a galaxy to fuel star formation.

The HDST’s unexcelled sharpness at ultraviolet and optical wavelengths would allow astronomers to see the stellar and nebulous contents of galaxies billions of light-years away with the same crispness that Hubble sees inside galaxies just tens of millions of light-years away. The HDST could pick out stars like our Sun located 30 million light-years away! A sharp view of visible contents of the entire universe would immediately become accessible to us via this super-Hubble’s “high-definition” vision.

A simulated spiral galaxy as viewed by Hubble, and the proposed High Definition Space Telescope (HDST) at a lookback time of approximately 10 billion years (z = 2) The renderings show a one-hour observation for each space observatory. Hubble detects the bulge and disk, but only the high image quality of HDST resolves the galaxy’s star-forming regions and its dwarf satellite. The zoom shows the inner disk region, where only HDST can resolve the star-forming regions and separate them from the redder, more distributed old stellar population.

Within our own solar system, HDST would provide images of weather and surfaces on the outer planets and their moons far beyond today’s capabilities. HDST would also provide detailed data on the interaction of each of the outer planets with the solar wind and give planetary scientists the ability to search for remote, hidden members of our solar system ranging in size from dwarf planets to ice giants like Neptune.

Though such a telescope is envisioned for the 2030s, it is not too early to start planning the science needs and technological requirements. Planning for the Hubble Space Telescope began in the 1970s, two decades before its launch. In addition, concept studies for the Webb telescope began two decades ago.

The HDST is needed to complement the powerful capabilities of a new generation of ground-based telescopes. Planned for the early 2020s are behemoth visible-infrared observatories, such as the Thirty Meter Telescope, the 39-meter European Extremely Large Telescope, and a planned Giant Magellan Telescope. Already in operation is the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope in northern Chile.

The HDST would be able to study extremely faint objects that are 10 to 20 times dimmer than anything that could be seen from the ground with the planned large, ground-based telescopes. It could also observe ultraviolet wavelengths that are blocked by Earth’s atmosphere. The large ground-based telescopes, in turn, would be as good or better than HDST for measuring the spectra of objects. The HDST would have comparable clarity at UV/optical wavelengths as the giant ground-based telescopes get in the near infrared and as ALMA gets at millimeter wavelengths. This would allow astronomers to obtain incredibly clear views of the cosmos over a very broad electromagnetic spectral range.