Astronomers have found clear evidence of a planet orbiting the closest star to Earth, Proxima Centauri. Proxima is a red dwarf. The planet is 1.3 Earth masses. It orbits at 0.05 AU. It lies squarely in the center of the classical habitable zone for Proxima.
What big telescopes are being built or are funded which will provide a better look a this and other earthlike exoplanets ?
* James Webb Space Telescope (2018)
* 24 meter ground based telescope, Giant Megallan (2022-2025)
* 39 meter ground based Extremely Large Telescope (2024-2027)
* 30 Meter Telescope (permit delays, around 2022-2025 if resolved)
* Proposed 12 meter High definition space telescope (could spot earth sized planets out to 45 light years and directly image Proxima B). If funded could get built by 2030 or so.
JWST will have the capability to detect key markers that could indicate the presence of a climate like our own when looking at Earth-sized planets around stars that are smaller and redder than our sun (like Proxima b). A study shows that the James Webb Space Telescope – Hubble’s successor could distinguish between an Earth and a Venus orbiting a cool, red star not too far away. But making that observation wouldn’t be easy.
The 24 meter Giant Magellan Telescope should also be operating around 2022-2025. The GMT will have absolute magnitude capability of 29.
Targets for direct imaging exoplanets fall into a few distinct classes:
• Planets still embedded in their parent disks (age = 1 – 10Myr, at 30 – 150pc).
• Young (0.1 – 1Gyr), nearby (3 – 50pc) gas-giant planets, which are intrinsically bright in the near-infrared due to their on-going gravitational contraction and,
• Older (over 1Gyr) planets detectable via their thermal infrared emission or reflected light.
The GMT will provide high contrast, high resolution imaging capabilities in the near and mid-infrared enabling the detection of exoplanets in each of these categories.
One of the technical goals of the GMT is to detect objects more than one million times fainter than the host star at angular separations corresponding to 1.5 λ/D to 20 λ/D
Extremely Large Telescope
The telescope’s segmented mirror will be 39.3 meters in diameter and will gather 15 times more light than the largest optical telescopes operating at the time of its development. The telescope has an innovative five-mirror design that includes advanced adaptive optics to correct for the turbulent atmosphere, giving exceptional image quality
Proposed High Definition Space Telescope
The High Definition Space Telescope is a proposed space telescope that would be five times as big and 100 times as sensitive as the Hubble, with a mirror nearly 40 feet in diameter, and would orbit the sun about a million miles from Earth.
The 10 milliarcsec resolution element of a 12 meter telescope (diffraction limited at 0.5 micron) would reach a new threshold in spatial resolution. It would be able to take an optical image or spectrum at about 100 parsec spatial resolution or better, for any observable object in the entire Universe. Thus, no matter where a galaxy lies within the cosmic horizon, we would resolve the scale at which the formation and evolution of galaxies becomes the study of their smallest constituent building blocks—their star-forming regions and dwarf satellites. Within the Milky Way, a 12 m telescope would resolve the distance between the Earth and the Sun for any star in the Solar neighborhood, and resolve 100 AU anywhere in the Galaxy. Within our own Solar System, we would resolve structures the size of Manhattan out at the orbit of Jupiter
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 because 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. Credit: L. Pueyo, M. N’Diaye (STScI).
The high definition space telescope is diffraction limited at 500nm, right in the middle of the visible spectrum. Diffraction limit is effectively the wavelength that any circular mirror gives its best angular resolution, the ability to discern detail. Angular resolution is governed by the equation λ (lambda) or wavelength expressed as a fraction of a meter / telescope aperture (D) expressed in metres; e.g HDST has its optimum functioning or “diffraction limit” at 500nm wavelength, defined by the equation 500nm (10-9)/12m.
The higher the aperture of a telescope the more detail it can see at any given wavelength and conversely the longer the wavelength, the less detail it can see. That is under perfect conditions experienced in space as opposed to the constantly moving atmosphere for ground-based scopes that will rarely approach the diffraction limit. So the HDST will not have the same degree of resolution at infrared wavelengths as visible wavelengths, which is relevant as several potential biosignatures will appear on spectra at longer wavelengths.
Approaching the diffraction limit is possible on the ground with the use of laser-produced guide stars and modern “deformable mirrors or “adaptive optics,” which help compensate. This technique of deformable primary and especially secondary mirrors will be important in space as well, in order to achieve the incredible stability required for any telescope observing distant and dim exoplanets. This is especially true of coronagraphs, though much less so with star-shades, which could be important in determining which starlight suppression technique to employ.
A large 12 meter HDST would require a WFE of about 1/20 lambda and possibly even lower, which works out to less than 30nm. The telescope would also require a huge giga-pixel array of sensors to capture any exoplanet detail, electron-magnifying CCDs, Electron Multiplying CCDs (EMCCDs), or their Mercury Cadmium Tellurium-based near infrared equivalent, which would need passive cooling to prevent heat generated from the sensors themselves producing “dark current,” creating a false digital image and background “noise”.
Such arrays already exist in space telescopes like the ESA Gaia, and producing larger versions would be one of the easier design requirements. For a UltraViolet-Optical-InfraRed (UVOIR) telescope an operating temperature of about -100 C would suffice (for the sensors, while only the telescope itself would be near room temperature).
All of the above is difficult but not impossible even today and certainly possible in the near future, with conventional materials like ultra-low expansion glass (ULE) able to meet this requirement, and more recently silicon carbide composites
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.
Credit: D. Ceverino, C. Moody, and G. Snyder, and Z. Levay (STScI).