The Hubble Telescope turned the Moon into a mirror. In this way, we will look for life on other planets


During a total lunar eclipse, astronomers using the Hubble Space Telescope discovered ozone in the Earth’s atmosphere. Using the same method, they would be able to search for life on Earth-like planets orbiting other stars.

These were the first observations of a total lunar eclipse with a space telescope, and the first observations of an ultraviolet eclipse.

To prepare for the study of exoplanets with much larger, emerging telescopes, astronomers decided to carry out a few experiments much closer, focusing their observations on the only known planet with life to date: Earth. The ideal alignment of the Earth, the Sun and the Moon during a total lunar eclipse allows you to study the Earth as you would study rocky planets passing in front of their host star.

A diagram showing the geometry of the lunar eclipse. Source: ESA / Hubble, M. Kornmesser

In this case, Hubble was not looking directly at Earth. Instead, astronomers used the Moon as a mirror to reflect the Sun’s rays as they passed through the Earth’s atmosphere. Using a space telescope to observe an eclipse, astronomers receive much more accurate data than from ground-based telescopes, because the collected data is not contaminated by the Earth’s atmosphere.

Observation was a challenge for Hubble

Just before the eclipse, the moon is very bright and its surface is not a perfect mirror as it is dotted with brighter and darker regions. Moreover, the Moon is so close to Earth that Hubble had to go to great lengths to keep looking at the same part of the surface all the time to track the Moon’s movement relative to the observatory.

During the measurements, the telescope registered a strong signal of ozone, an element necessary for the existence of life as we know it. While ozone signatures have already been detected with ground-based telescopes during eclipses, Hubble observations gave the strongest signal for this molecule, as Hubble can observe ultraviolet radiation that is absorbed by our atmosphere and does not reach the Earth’s surface.

The processes of photosynthesis taking place over billions of years have made the Earth’s atmosphere contain a lot of oxygen and have a thick ozone layer. It was only 600 million years ago that the Earth’s atmosphere collected enough ozone to effectively protect life from the harmful ultraviolet radiation emitted by the Sun. Only then did life begin to emerge from the oceans to the surface of the lands.

The discovery of ozone in the spectrum of an exoplanet would be a significant discovery, as it is a photochemical side effect of the presence of molecular oxygen, which in turn is emitted by living organisms, explains Alison Youngblood of the Colorado Atmospheric and Space Physics Laboratory.

Hubble recorded the spectral signature of ozone in ultraviolet light in sunlight filtered through the Earth’s atmosphere during the lunar eclipse of January 20-21, 2019. At the same time, several other telescopes also made spectroscopic observations at various wavelengths, looking for such components in the Earth’s atmosphere such as oxygen, methane, water and carbon monoxide.

Our research clearly demonstrates the benefits of ultraviolet spectroscopy for researchers who will describe exoplanets. – says Antonio Garcia Munoz from the Technical University of Berlin.

The atmospheres of some exoplanets can be studied when the planets pass in front of their star in the course of a so-called transit. During transit, the light emitted by the star passes through the atmosphere of the exoplanet. Chemical compounds present in the atmosphere of such a planet leave characteristic traces in the starlight. So far, researchers have tried such observations, but they concerned huge gas giants whose atmospheres are vast and very thick. In the case of earth-like rocky planets, making such observations is much more difficult because the planet itself is much smaller and its atmosphere is usually extremely thin.

For this reason, researchers will need space telescopes much larger than Hubble to even think of collecting the delicate light passing through the atmospheres of small planets in distant planetary systems.

However, the discovery of ozone in a rocky exoplanet’s atmosphere will not automatically mean that there is life on its surface.

To make such a statement, we will also need the spectral signatures of other compounds, and they are no longer visible in the ultraviolet range – adds Youngblood.

The image shows the section of the Moon where the Hubble Space Telescope was staring as it measured the amount of ozone in the Earth’s atmosphere.

Astronomers will have to look for combinations of biosignatures such as ozone and methane, looking for signs of life on other planets. Here, too, it is worth remembering that, for example, ozone accumulates in the atmosphere over hundreds of millions of years. Just 2 billion years ago, there was much less ozone in the Earth’s atmosphere.

And now a sentence that has been appearing in this type of article for many years: the James Webb Space Telescope, which is slated to launch next year, will be able to look deep into the atmospheres of planets and detect the presence of methane and oxygen in them.

We expect JWST to significantly shift the frontiers of transmission spectroscopy in the study of exoplanetary atmospheres, says Garcia Munoz.

Importantly, it will be able to discover methane and oxygen in the atmospheres of planets orbiting nearby small stars.

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The Hubble ‘telescope turned the moon into a mirror. In this way, we will look for life on other planets