We have only one example of biology forming in the universe – life on Earth. But what if life can form in other ways? How do you look for alien life when you don’t know what alien life might look like?

These questions are preoccupying astrobiologists, who are scientists who look for life beyond Earth. Astrobiologists have attempted to come up with universal rules that govern the emergence of complex physical and biological systems both on Earth and beyond.

I’m an astronomer who has written extensively about astrobiology. Through my research, I’ve learned that the most abundant form of extraterrestrial life is likely to be microbial, since single cells can form more readily than large organisms. But just in case there’s advanced alien life out there, I’m on the international advisory council for the group designing messages to send to those civilizations.

Detecting life beyond Earth

Since the first discovery of an exoplanet in 1995, over 5,000 exoplanets, or planets orbiting other stars, have been found.

Many of these exoplanets are small and rocky, like Earth, and in the habitable zones of their stars. The habitable zone is the range of distances between the surface of a planet and the star it orbits that would allow the planet to have liquid water, and thus support life as we on Earth know it.

The sample of exoplanets detected so far projects 300 million potential biological experiments in our galaxy – or 300 million places, including exoplanets and other bodies such as moons, with suitable conditions for biology to arise.

The uncertainty for researchers starts with the definition of life. It feels like defining life should be easy, since we know life when we see it, whether it’s a flying bird or a microbe moving in a drop of water. But scientists don’t agree on a definition, and some think a comprehensive definition might not be possible.

NASA defines life as a “self-sustaining chemical reaction capable of Darwinian evolution.” That means organisms with a complex chemical system that evolve by adapting to their environment. Darwinian evolution says that the survival of an organism depends on its fitness in its environment.

The evolution of life on Earth has progressed over billions of years from single-celled organisms to large animals and other species, including humans.

Exoplanets are remote and hundreds of millions of times fainter than their parent stars, so studying them is challenging. Astronomers can inspect the atmospheres and surfaces of Earth-like exoplanets using a method called spectroscopy to look for chemical signatures of life.

Spectroscopy might detect signatures of oxygen in a planet’s atmosphere, which microbes called blue-green algae created by photosynthesis on Earth several billion years ago, or chlorophyll signatures, which indicate plant life.

NASA’s definition of life leads to some important but unanswered questions. Is Darwinian evolution universal? What chemical reactions can lead to biology off Earth?

Evolution and complexity

All life on Earth, from a fungal spore to a blue whale, evolved from a microbial last common ancestor about 4 billion years ago.

The same chemical processes are seen in all living organisms on Earth, and those processes might be universal. They also may be radically different elsewhere.

In October 2024, a diverse group of scientists gathered to think outside the box on evolution. They wanted to step back and explore what sort of processes created order in the universe – biological or not – to figure out how to study the emergence of life totally unlike life on Earth.

Two researchers present argued that complex systems of chemicals or minerals, when in environments that allow some configurations to persist better than others, evolve to store larger amounts of information. As time goes by, the system will grow more diverse and complex, gaining the functions needed for survival, through a kind of natural selection.

They speculated that there might be a law to describe the evolution of a wide variety of physical systems. Biological evolution through natural selection would be just one example of this broader law.

In biology, information refers to the instructions stored in the sequence of nucleotides on a DNA molecule, which collectively make up an organism’s genome and dictate what the organism looks like and how it functions.

If you define complexity in terms of information theory, natural selection will cause a genome to grow more complex as it stores more information about its environment.

Complexity might be useful in measuring the boundary between life and nonlife.

However, it’s wrong to conclude that animals are more complex than microbes. Biological information increases with genome size, but evolutionary information density drops. Evolutionary information density is the fraction of functional genes within the genome, or the fraction of the total genetic material that expresses fitness for the environment.

Organisms that people think of as primitive, such as bacteria, have genomes with high information density and so appear better designed than the genomes of plants or animals.

A universal theory of life is still elusive. Such a theory would include the concepts of complexity and information storage, but it would not be tied to DNA or the particular kinds of cells we find in terrestrial biology.

Implications for the search for extraterrestial life

Researchers have explored alternatives to terrestrial biochemistry. All known living organisms, from bacteria to humans, contain water, and it is a solvent that is essential for life on Earth. A solvent is a liquid medium that facilitates chemical reactions from which life could emerge. But life could potentially emerge from other solvents, too.

Astrobiologists Willam Bains and Sara Seager have explored thousands of molecules that might be associated with life. Plausible solvents include sulfuric acid, ammonia, liquid carbon dioxide and even liquid sulfur.

Alien life might not be based on carbon, which forms the backbone of all life’s essential molecules – at least here on Earth. It might not even need a planet to survive.

Advanced forms of life on alien planets could be so strange that they’re unrecognizable. As astrobiologists try to detect life off Earth, they’ll need to be creative.

One strategy is to measure mineral signatures on the rocky surfaces of exoplanets, since mineral diversity tracks terrestrial biological evolution. As life evolved on Earth, it used and created minerals for exoskeletons and habitats. The hundred minerals present when life first formed have grown to about 5,000 today.

For example, zircons are simple silicate crystals that date back to the time before life started. A zircon found in Australia is the oldest known piece of Earth’s crust. But other minerals, such as apatite, a complex calcium phosphate mineral, are created by biology. Apatite is a primary ingredient in bones, teeth and fish scales.

Another strategy to finding life unlike that on Earth is to detect evidence of a civilization, such as artificial lights, or the industrial pollutant nitrogen dioxide in the atmosphere. These are examples of tracers of intelligent life called technosignatures.

It’s unclear how and when a first detection of life beyond Earth will happen. It might be within the solar system, or by sniffing exoplanet atmospheres, or by detecting artificial radio signals from a distant civilization.

The search is a twisting road, not a straightforward path. And that’s for life as we know it – for life as we don’t know it, all bets are off.

Chris Impey, University Distinguished Professor of Astronomy, University of Arizona

This article is republished from The Conversation under a Creative Commons license. Read the original article.

In late November 2024, however, California regulators recalled two batches of raw, unpasteurized milk from a Fresno dairy farm after bird flu virus was detected in the milk. The dairy subsequently recalled all of its raw milk and cream products from stores due to possible bird flu contamination. State regulators placed the farm under quarantine, suspending any new distribution of its raw milk, cream, kefir, butter and cheese products produced on or after November 27.

No human bird flu cases associated with the milk were detected immediately following the recalls. But officials strongly urged buyers not to drink raw milk from the affected batches and to return it to the store where they bought it.

Despite health experts’ warning that raw milk could contain high levels of the avian flu virus, along with many other pathogens, raw milk sales are up in the U.S. Robert F. Kennedy Jr., whom President-elect Donald Trump has said he will nominate to head the Department of Health and Human Services, has asserted that raw milk “advances human health,” contrary to warnings from FDA officials and food scientists.

As an extension food scientist in a state where raw milk sales are legal, I provide technical support to help processors produce high-quality, safe dairy foods. I also like to help people understand the confusing world of pasteurization methods on their milk labels, and why experts strongly discourage consuming raw milk and products made from it.

What can make milk unsafe

Dairy products, like many foods, have inherent risks that can cause a variety of illnesses and even death. Dairy milk comes from animals that graze outdoors, live in barns and lie in mud and manure. Milk is picked up from the farm in tanker trucks and delivered to the processing plant. These environments offer numerous opportunities for contamination by pathogens that cause illness and produce organisms that make food spoil.

For example, listeria monocytogenes comes from environmental sources like soil and water. Mild infections with listeriosis cause flu-like symptoms. More serious cases are, unfortunately, too common and can cause miscarriages in pregnant women and even death in extreme cases.

Other pathogens commonly associated with dairy animals and raw milk include E. coli, which can cause severe gastrointestinal infections and may lead to kidney damage; Campylobacter, the most common cause of diarrheal illness in the U.S.; and Salmonella, which cause abdominal pain, diarrhea and other symptoms.

Keeping beverages safe with heat

In the 1860s, French microbiologist Louis Pasteur discovered that heating wine and beer killed the organisms that caused spoilage, which then was a significant problem in France.

This heating process, which became known as pasteurization, was adopted in the U.S. prior to World War II, at a time when milk was responsible for 25% of all U.S. outbreaks of foodborne illnesses. In 1973 the federal government required that all milk sold across state lines in the U.S. had to be pasteurized, and in 1987 it banned interstate sales of raw milk.

Pasteurization heats every particle of a food to a specific temperature for a continuous length of time in order to kill the most heat-resistant pathogen associated with that product. Different organisms have different responses to heat, so controlled scientific studies are required to determine what length of time at a given temperature will kill a specific organism.

Since 1924, pasteurization in the U.S. has been guided by the Grade “A” Pasteurized Milk Ordinance, a federal guidance document that is updated every two years to reflect current science and has been adopted by all 50 states. Pasteurization equipment in the U.S. must meet stringent requirements that include sanitary design, safety controls and material standards.

Pasteurization methods

Dairy processors can choose among several different types of pasteurization. When executed properly, all of these methods produce the same result: pathogen-free milk. Processors may treat milk beyond minimum times or temperatures to provide an extra margin of safety, or to reduce bacteria that can cause milk to spoil, thus increasing the product’s shelf life.

Smaller-scale processors who handle limited volumes use what are known as vat pasteurizers, also known as batch pasteurizers. Milk is pumped into a temperature-controlled tank with a stirrer, heated to a minimum of 145 degrees Fahrenheit (63 Celsius) and held there continuously for 30 minutes. Then it is cooled and pumped out of the vat.

The most common method used for commercial milk is high-temperature short-time pasteurization, which can treat large volumes of milk. Milk is pumped through a series of thin plates at high speed to reach a minimum temperature of 161 F (71 C). Then it travels through a holding tube for 15 seconds, and the temperature is checked automatically for safety and cooled.

The most complex and expensive systems are ultra-pasteurizers and ultra-high-temperature pasteurizers, which pasteurize milk in just a few seconds at temperatures above 285 F (140 C). This approach destroys many spoilage organisms, giving the milk a significantly longer shelf life than with other methods, although sometimes products made this way have more of a “cooked” flavor.

Ultra-high-temperature products are processed in a sterile environment and packaged in sterile packaging, such as lined cartons and pouches. They can be shelf-stable for up to a year before they are opened. Ultra-high-temperature packaging makes taking milk to school for lunch safe for kids every day.

Avian flu in milk

The detection of avian flu virus fragments in milk is a new challenge for the dairy industry. Scientists do not have a full picture of the risks to humans but are learning.

Research so far has shown that virus particles end up in the milk of infected cows, but that pasteurization will inactivate the virus. The FDA advises consumers not to drink raw milk because there is limited information about whether it may transmit avian flu.

The agency also is urging producers not to manufacture or sell raw milk or raw milk products, including cheese, made with milk from cows showing symptoms of illness.

Avian flu continues to appear in new species, and as of early December 2024, 57 human cases had been confirmed in the U.S. Of these, all but two were people who worked with livestock.

Two recent cases – a child in California and a teen in Canada – may indicate that young people with immature immune systems are more vulnerable than adults to the virus. With medical researchers still learning how H5N1 is transmitted, I agree with the FDA that raw milk poses risks not worth taking.

This is an updated version of an article originally published on May 17, 2024.

Kerry E. Kaylegian, Associate Research Professor of Food Science, Penn State

This article is republished from The Conversation under a Creative Commons license. Read the original article.