Despite having discovered thousands of exoplanets, we have not detected convincing evidence of alien life on any of them. But astronomy is reaching a point where powerful telescopes can zoom in on many more potentially habitable worlds. Planets that orbit M dwarfs, which are small stars cooler than the sun, are particularly promising targets because they have a unique observation window. When a planet passes in front of one of these stars, it blocks a relatively large fraction of the star's light, making the planet and its atmosphere easier to detect.

There are also a lot of them. M dwarfs are the most common type of star in the Milky Way. However, if we misunderstand how these stars interact with planetary atmospheres, we may misinterpret the significance of observations from some of the most promising planets.

The most-studied biosignatures are methane, oxygen and ozone. On Earth, these are often produced by biological processes, with ozone acting as an indirect indicator of oxygen. But even on our home planet, molecules typically associated with life are not unambiguous biological fingerprints. These molecules can be produced by geological and chemical processes, as well as via atmospheric reactions with light from the sun.

A potential biosignature detection found elsewhere in the cosmos is therefore a clue, not proof of life. We can't interpret the chemical signals coming from a potentially habitable planet without understanding the star that illuminates it. M dwarfs produce ultraviolet radiation that can break apart molecules and trigger chemical reactions in the atmosphere of an orbiting planet. The intensity and wavelength of the radiation influence which molecules form and survive, as well as how abundant they become. Two planets with otherwise identical properties could develop very different atmospheres simply because they orbit stars with different ultraviolet emissions. Radiation from a star could therefore make the same level of biological activity appear stronger on one planet than on another, or make nonbiological chemistry look like life. A recent study submitted to the preprint server arXiv Aug. 19 demonstrated this using entirely simulated planets. The researchers, led by University of California, Santa Cruz astronomy graduate student C. Evan Davis, simulated Earth-like planets orbiting two different types of M dwarf with ages ranging from 650 million to 5 billion years.

The team considered atmospheres resembling that of Earth during the oxygen-rich preindustrial era and the Archean eon (about 4 billion to 2.5 billion years ago), during which the first life-forms emerged but atmospheric oxygen was scarce. The models considered the stars' usual, or "quiescent," ultraviolet emission rather than short-lived flares. By changing the modeled stars' ages and ultraviolet radiation while keeping the planets comparable, they investigated how a star's evolution changed an atmosphere and the signals astronomers might observe.

One of the clearest differences the team found appeared in methane. Simulated planets with preindustrial atmospheres orbiting 5 billion-year-old M dwarfs accumulated up to 10 times more methane than equivalent planets simulated to orbit the younger 650 million-year-old stars. The methane signals produced in the simulated data were up to 68% stronger for the older systems. The weaker UV emission from the older M dwarf allowed methane to survive longer and accumulate in the simulated planetary atmospheres. The stronger methane signal could make a planet orbiting an older star appear to support more biological activity, when this difference was actually caused by the UV emission of the host star.

An even more striking result came from ozone. For the simulated Archean Earth-like planets, which were low in oxygen and rich in carbon dioxide (CO₂), the stronger UV radiation from younger M dwarfs broke apart more CO₂. This kick-started reactions that created oxygen and ozone without any life involved. Some model planetary atmospheres contained more than 100,000 times as much ozone as equivalent models around older stars. A hypothetical observer could misinterpret the resulting ozone signal as indirect evidence of biologically produced oxygen. Ozone is not useless as a biosignature, but these simulations show that it cannot be interpreted confidently without knowing the UV environment that shaped the atmosphere.

Of course, astronomers are already aware that a single molecule would not prove the existence of life on an exoplanet. Researchers use atmospheric models, look for combinations of gases, and consider nonbiological explanations before describing a signal as a potential biosignature. These methods remain valuable for identifying the most promising planets for further investigation, even when our knowledge of their stars is incomplete. Some might argue that these methods provide a sufficiently reliable first assessment and we should reserve more detailed stellar observations for the strongest candidates.

But even the best atmospheric model can mislead us if the stellar radiation isn't accounted for correctly. Ultraviolet observations of M dwarfs remain limited, so researchers often rely on estimates from similar stars. However, two M dwarfs that might appear similar can produce very different levels of UV radiation. Atmospheric models based on currently available stellar measurements may be sufficient for selecting promising targets but not for deciding whether the origin of a signal is biological. Simply acknowledging that the star matters isn't enough; we need accurate information about the specific star hosting the planet.

So before we train our telescopes on promising exoplanets, we need to study their host stars. This requires repeated UV observations, typically with different telescopes or instruments or at different time periods than you'd use to study the exoplanet. These observations can characterize how the host star's emission varies over time, while better stellar-age estimates — which can be calculated from properties such as rotation and magnetic activity— will reveal how that radiation has evolved.

Then, once we do start studying an exoplanet, potential biosignatures must be analyzed and interpreted using models informed by accurate measurements of the host star. As recommended in the preprint study, astronomers should also search for accompanying molecules, such as carbon monoxide, that could reveal whether ozone arose through reactions between light and carbon dioxide rather than from biology. The first convincing discovery of life on another planet will depend on an understanding of not only that planet but also the star that shaped the atmosphere we observe.

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'> The hunt for extraterrestrial life is fundamentally flawed. But there's a fix.

Archaeologists think the hoard was buried at the time of the Emperor Hadrian, who ruled from A.D. 117 to 138, because the most recent coin was minted in 124 or 125.

But the coins span more than three centuries: the oldest was minted in 148 B.C., during the time of the Roman Republic; while most of them are from the time of the Flavian emperors — Vespasian and his sons, Titus and Domitian — from A.D. 69 to 96.

Silver signal

The statement praised the metal detectorist, Oliver Riedl, who found the hoard and stopped digging after unearthing 15 of the coins. At that point, his metal detector signaled that something much larger was hiding underground.

"It was clear to me that there was something bigger in the ground here," Riedl said in the statement. "I therefore immediately informed the office."

Riedl is licensed by the state to use his metal detector and has been trained by regional archaeological authorities, the statement said. (Illegal metal detectorists also operate in Germany, but are known to often disrupt the archaeological value of their finds).

Archaeologists from the Rhineland regional government soon investigated the find and excavated the entire hoard of 934 Roman silver coins. The block of earth that contained the coin hoard was removed whole and X-rayed before it was opened, the statement noted.

Erich Claßen, the head of archaeology for the Rhineland regional government, said it seemed that the original owner collected most of the coins in about A.D. 100, but didn't actually bury them until Hadrian's time.

"We can only speculate about the motives, but very likely someone wanted to keep this money safe," he said in the statement. "Banks did not exist at that time."

But whoever buried it never came back. "It is possible that the owner died beforehand without being able to pass on the knowledge of the coins," Claßen said.

Buried treasure

Marjanko Pilekić, a numismatist, or coin expert, at the German Historical Museum in Berlin who wasn't involved in the find, said it was significant that the hoard was unearthed just inside the northern frontier of the Roman Empire demarcated by the Roman limes, which ran along the Rhine nearby.

He told Live Science in an email that the location suggested when the hoard had been purposefully buried: "In this case, we can cautiously assume that the hoard was buried not long after the most recent coin was minted," Pilekić said.

Outside the Roman Empire, however, in the Barbaricum – the "barbarian" regions beyond, according to the Romans — such hoards of coins were often buried much later, he said.

While its historical value is priceless, the worth of such a hoard during the second century A.D. would have been considerable: 300 denarii was the yearly pay of a Roman legionary at the time of Hadrian, of which around half was deducted for food and equipment, regional archaeologist Rahel Otte, who led the excavation, said in the statement.

This means that a legionary "would have had to save all his available money for about six years to accumulate the Wesseling treasure find," she said.

She added that it was unclear who had buried the hoard, but it may have been someone who lived on one of the Roman estates in the area.

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'> 'There was something bigger in the ground here': Metal detectorist in Germany discovers giant Roman-era hoard from time of emperor Hadrian

The agreement, known as the Treaty of Kadesh, was signed around 1269 B.C. by Egyptian pharaoh Ramesses II and Hittite king Hattusili III. Other parts of the treaty have already been found, but the new fragment was unearthed during an excavation at Hattusha, in north-central Turkey.

"We have found traces of peace, thousands of years old, in the soil of Anatolia," Mehmet Nuri Ersoy, Turkey's minister of culture and tourism, said in a Sept. 21 translated statement. The newly discovered cuneiform tablet fragment contains text from a section of the treaty related to the treatment of refugees, according to the statement. "It reminds us of our responsibility to protect human life and dignity," Ersoy said.

Archaeologists unearthed the fragment from a building in the ancient Hittite capital of Hattusha. The Hittite Empire controlled central Anatolia from about 1650 to 1200 B.C. during the Bronze Age. Archaeologists have discovered numerous examples of diplomatic documents at Hattusha that were written in Akkadian, the earliest documented Semitic language, including other fragments of the Treaty of Kadesh.

The royal leaders signed the treaty around 1269 B.C. to mark the end of a lengthy war between the ancient Egyptians and the Hittite Empire, and it is regarded as the world's oldest known peace treaty. Although known as the Treaty of Kadesh, the document does not actually mention the Battle of Kadesh, which occurred several years earlier, so it is also called the Egyptian-Hittite peace treaty, the Eternal Treaty or the Silver Treaty.

The treaty was preserved by inscriptions in two ancient cities. In the early 19th century, archaeologists first discovered the treaty inscribed in hieroglyphics on the walls of a temple dedicated to Ramesses II in Thebes (modern Luxor). Fragments of the Akkadian cuneiform text of the treaty were first identified at Hattusha in the early 20th century.

Marking the end of a two-century-long war over lands in the eastern Mediterranean, the Treaty of Kadesh marks a military détente between Ramesses II and Hattusili III. According to a translation of the treaty provided by the Turkish Ministry of Culture and Tourism, it includes lines like, "He is a brother to me and he is at peace with me; and I am a brother to him and I am forever at peace with him," referring to the two leaders. "The country of Egypt and the country of Hatti [Hittite Empire] will be forever in a state of peace and of fraternity as it is with us."

The treaty has further provisions for coming to a country's aid if it was invaded and on how to treat refugees. The text mentions that any refugee from Egypt to Hattusha will be returned to Egypt, and any Hattusha refugee to Egypt will be returned to Hattusha, as fleeing was considered a crime.

However, the treaty specifically forbids punishing refugees: "Their tongue and their eyes are not to be pulled out; their ears and their feet are not to be cut off; their houses with their wives and their children are not to be destroyed." Part of this text appears on the newly discovered cuneiform fragment. Ongoing excavations may help uncover the remaining parts of the treaty, according to the statement.

A copper replica of the Treaty of Kadesh is displayed prominently today in the United Nations Conference Building in New York City. According to the U.N., "the treaty pledges eternal friendship, lasting peace, territorial integrity, nonaggression, extradition, and mutual help. The pledges in this treaty are similar to the United Nations ideals."

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'> Fragment of world's oldest peace treaty unearthed in Turkey