- 2814 - HUBBLE - tests to detect exoplanets life? Astronomers have been using a variety of ground- and space-based telescopes to analyze how the ingredients of Earth's atmosphere look from space, using our planet as a proxy for studying extrasolar planets' atmospheres. They hope to eventually compare Earth's atmospheric composition with those of other worlds to note similarities and differences.
--------------- 2814 - HUBBLE - tests to detect exoplanets life?
- When Earth is viewed from space astronauts have been awestruck at our blue marble planet's majesty and diversity. Our planet is a paradise. We are very lucky to be here.
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- What's mind-blowing is that astronomers also estimate there could be as many as 1 billion other planets like Earth in our Milky Way galaxy alone. Just imagine, one billion – not million but a billion other "paradise planets."
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- But it's paradise lost if nothing is living there.
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- It is sobering that our home planet is the only known place in the universe where life as we know it exists and thrives. And so, we gaze outward to the stars, imprisoned by space and time, into a cosmic loneliness.
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- Scientists are dedicated to building ever-larger telescopes to search for potentially habitable planets. But how will they know life is present without traveling there and watching creatures walk, fly, or slither around?
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- One way to find a habitable planet is by probing the planet's atmosphere. An atmosphere with the right mix of chemical elements is necessary to nurture and sustain life as we know it.
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- Earth's atmosphere includes oxygen, nitrogen, methane, and carbon dioxide that have helped support life for billions of years. Earth's abundance of oxygen, especially, is a clue that our atmosphere's oxygen content is always being replenished by other biological processes.
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- Taking advantage of a total lunar eclipse, astronomers using the Hubble telescope have detected ozone in Earth's atmosphere by looking at “Earthlight” reflected off the Moon. Our Moon came in handy as a giant mirror in space.
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- Ozone is a key ingredient in our planet's atmosphere. It forms naturally when oxygen is exposed to strong concentrations of ultraviolet light, which triggers the chemical reactions. Ozone is also Earth's security blanket, protecting life from deadly ultraviolet rays. Be thankful for our ozone layer.
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- The Hubble experiment was the first time a total lunar eclipse was captured at ultraviolet wavelengths and from a space telescope. This same method simulates how astronomers will search for evidence of life beyond Earth by looking for potential biosignatures on extrasolar planets.
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- Using a space telescope for eclipse observations reproduces the conditions under which future telescopes would measure atmospheres of extrasolar planets that pass in front of their stars. These atmospheres may contain chemical signatures very similar to Earth, and further our curiosity to wonder if we are not alone in the universe.
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- Hubble Telescope did not look at Earth directly. Instead, the astronomers used the Moon as a mirror to reflect sunlight, which had passed through Earth's atmosphere, and then reflected back towards Hubble.
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- Using this space telescope for these eclipse observations reproduces the conditions under which future telescopes would measure atmospheres of transiting exoplanets. These observations may find chemicals of interest to astrobiology.
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- Though numerous ground-based observations of this kind have been done previously, this is the first time a total lunar eclipse was captured at ultraviolet wavelengths and from a space telescope.
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- The Hubble observation detected the strong spectral fingerprint of ozone, which absorbs some of the sunlight and is the source of the protective shield in Earth's atmosphere.
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- On Earth, photosynthesis over billions of years is responsible for our planet's high oxygen levels and thick ozone layer. That's one reason why scientists think ozone or oxygen could be a sign of life on another planet. Ozone is significant is a photochemical byproduct of molecular oxygen, which is itself a byproduct of life.
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- Hubble recorded ozone absorbing some of the Sun's ultraviolet radiation that passed through the edge of Earth's atmosphere during a lunar eclipse that occurred on January 20 to 21, 2019. Several other ground-based telescopes also made spectroscopic observations at other wavelengths during the eclipse. They were searching for more of Earth's atmospheric ingredients, such as oxygen and methane.
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- The atmospheres of some extrasolar planets can be probed if the alien world passes across the face of its parent star, an event called a transit. During a transit, starlight filters through the backlit exoplanet's atmosphere. If viewed close up, the planet's silhouette would look like it had a thin, glowing "halo" around it caused by the illuminated atmosphere, just as Earth does when seen from space.
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- Chemicals in the atmosphere leave their telltale signature by filtering out certain colors of starlight. So far, astronomers have used Hubble to observe the atmospheres of gas giant planets and super-Earths (planets several times Earth's mass) that transit their stars.
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- But terrestrial planets about the size of Earth are much smaller objects and their atmospheres are thinner, like the skin on an apple. Therefore, teasing out these signatures from Earth-sized exoplanets will be much harder.
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- That's why researchers will need space telescopes much larger than Hubble to collect the feeble starlight passing through these small planets' atmospheres during a transit. These telescopes will need to observe planets for a longer period, many dozens of hours, to build up a strong signal.
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- To prepare for these bigger telescopes, astronomers decided to conduct experiments on a much closer and only known inhabited terrestrial planet: Earth. Our planet's perfect alignment with the Sun and Moon during a total lunar eclipse mimics the geometry of a terrestrial planet transiting its star.
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- But the observations were also challenging because the Moon is very bright, and its surface is not a perfect reflector because it is mottled with bright and dark areas. The Moon is also so close to Earth that Hubble had to try and keep a steady eye on one select region, despite the Moon's motion relative to the space observatory. So they had to account for the Moon's drift in their analysis.
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- Photosynthesis might be the most productive metabolism that can evolve on any planet, because it is fueled by energy from starlight and uses cosmically abundant elements like water and carbon dioxide. These necessary ingredients should be common on habitable planets.
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- Seasonal variability in the ozone signature also could indicate seasonal biological production of oxygen, just as it does with the growth seasons of plants on Earth.
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- But ozone can also be produced without the presence of life when nitrogen and oxygen are exposed to sunlight. To increase confidence that a given biosignature is truly produced by life, astronomers must search for combinations of biosignatures. A multiwavelength campaign is needed because each of the many biosignatures are more easily detected at wavelengths specific to those signatures.
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- Astronomers will also have to take the developmental stage of the planet into account when looking at younger stars with young planets. If you wanted to detect oxygen or ozone from a planet similar to the early Earth, when there was less oxygen in our atmosphere, the spectral features in optical and infrared light aren't strong enough.
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- We think Earth had low concentrations of ozone before the mid-Proterozoic geological period which was between roughly 2.0 billion to 0.7 billion years ago when photosynthesis contributed to the build up of oxygen and ozone in the atmosphere to the levels we see today.
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- The ultraviolet-light signature of ozone features is very strong. Therefore ultraviolet may be the best wavelength for detecting photosynthetic life on low-oxygen exoplanets.
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- NASA has a forthcoming observatory called the James Webb Space Telescope that could make similar kinds of measurements in infrared light, with the potential to detect methane and oxygen in exoplanet atmospheres.
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- The James Webb Telescope is scheduled to launch in 2021. Several other reviews are available about this space mission. Note that astronomy has broadened it view beyond the visible wavelengths into the ultraviolet and into the infrared. Both new eyes are seeing new discoveries.
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- September 4, 2020 2814
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