The telescope reveals how we might find traces of life on other planets as well.

A lunar eclipse took place in January 2019. Because the earth stood between the moon and the sun, there was no direct sunlight on the moon for a while. That is not to say that the moon was completely dark; sunlight that had first trickled through outer regions of our Earth’s atmosphere still managed to make its way to the moon. And scientists have now scrutinized that light using the Hubble Space Telescope. And found so strong indications that the earth is home to life.

Importance of the research You
wouldn’t think that this would be big news right away. But in a way it is. For experiments like this one pave the way for future studies in which much more powerful telescopes, hunting for extraterrestrial life, will examine Earth-like planets in other galaxies. “One of NASA’s important goals is to identify planets that can harbor life,” said researcher Allison Youngblood. “But how do you know whether a planet is inhabited or uninhabited? What would (an inhabited planet, ed.) Look like if we studied it with our current techniques to characterize the atmosphere of planets? ” To find out, Youngblood and colleagues used Hubble to take a closer look at the only inhabited planet we know of.

Ozone
The earth’s atmosphere contains many chemicals. And these leave their mark on the sunlight that seeps through the atmosphere, for example by absorbing the sunlight at certain wavelengths. By studying the sunlight that has seeped through the atmosphere of a planet, researchers can therefore draw conclusions about which chemicals are present in an atmosphere. In this case, Hubble studied at ultraviolet wavelengths the sunlight that first seeped through the edge of our atmosphere and was reflected back into the telescope by the moon. In the sunlight, Hubble found a strong indication of the presence of ozone: a chemical that absorbs ultraviolet light very strongly.It is not the first time that researchers have used the moon to detect ozone in the Earth’s atmosphere. Earlier, telescopes already established on Earth were used during lunar eclipses to do this. And successfully. But the signal that ozone leaves in the sunlight captured by Hubble is much more powerful than the signal observed by Earth-based telescopes. This is because Hubble – located in space – is not affected by other chemicals present in Earth’s atmosphere during the observations.

Biosignature
That Hubble has succeeded in detecting such a powerful trace of ozone is promising. This is because ozone is a so-called biosignature. A chemical that – when found in a planet’s atmosphere – can actually indicate the presence of life. For example, the high concentration of ozone in the earth’s atmosphere can be traced back to life on its surface. “Finding ozone is significant because it is a photochemical byproduct of molecular oxygen, which is a byproduct of life,” said Youngblood. Ozone is one of the elements that researchers hope to discover in the atmosphere of other Earth-like planets.

Multiple wavelengths
Should that succeed, however, it is not immediately proven that there is life on such a planet. This is because ozone can also arise in the absence of life. To be sure that an Earth-like planet with ozone in the atmosphere is home to life, you actually have to detect multiple biosignatures. The problem, however, is that different chemicals absorb sunlight at different wavelengths. That means that in the search for life, you should ideally study the sunlight that seeps through the atmosphere of an Earth-like planet at multiple wavelengths.

Development phase
At the same time, you also have to take into account the development phase a planet is in. For example, it is believed that more than 2 billion years ago the earth was home to little ozone. It was only later, when more and more organisms started to photosynthesize, pumping oxygen into the atmosphere en passant, that the concentration of ozone – which is created when oxygen is exposed to ultraviolet sunlight – rose. Had aliens spotted our planet more than 2 billion years ago and searched for traces of life using techniques similar to the one we have today, their chances of success would have been greatest if they had looked at ultraviolet wavelengths, says researcher Giada Arney. “Because the signal from ozone at ultraviolet wavelengths is so powerful, you would expect to be able to detect small amounts of ozone in this way. Ultraviolet may therefore be the best wavelength to detect photosynthetic life on oxygen-deficient planets. ”

While Hubble has now successfully found a trail of life in Earth’s atmosphere, the telescope is probably less suitable for characterizing the atmosphere of Earth-like exoplanets. Previously, the space telescope – which was already launched years before the first exoplanet was discovered – has already been successfully used to inspect the atmosphere of super-Earths and gas giants. But because Earth-like planets are much smaller and their atmospheres much thinner, we need even more powerful telescopes for that. The good news is that these are already under development. For example , NASA hopes to – finally – launch the James Webb telescope next year. The telescope is extremely suitable for characterizing the atmosphere of Earth-like exoplanets and thanks to the work of Hubble we now get a bit of an idea of ​​what such an atmosphere looks like when a planet actually contains life.

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