The Search for the Cradle of Life at low radio frequencies

The question “Are we alone in the Universe?” is something humanity has pondered for centuries. Our attempts to answer may have started with trying to communicate with people on the moon but now it has taken us to the stars. 

Before we can determine how to search for life we have to first define what we are looking for.  This may be a seemingly obvious task but defining what is life and what would life look like are deceptively hard questions to answer.  The Oxford dictionary defines life as “the condition that distinguishes animals and plants from inorganic matter, including the capacity for growth, reproduction, functional activity, and continual change preceding death.”  So do we include microbes and single-cell organisms in this definition?

The current search for life and the understanding of what we are looking for crosses many fields of science; including astronomy, chemistry, physics, and biology.  By searching the most extreme environments of Earth we can see where life still manages to survive under these conditions.

We can use models and mathematics to try to predict what properties of stars are more likely to have planets that could sustain life as well.  For example, most planets we find are around variable stars which are far more active than our own Sun.  So does this mean that the atmosphere of a planet, if it had one, would continuously be changing, making it hard for life to adapt?  In a paper by Venot O., Rocchetto M et al, published in 2016 suggests that is not a problem.  In contrast, the recent work by Zic et al suggests that the star's activity can be stronger than we originally thought.

We can think of breaking the search for life in two main categories.  In one category, we assume that life is at least as advanced as our own where they are sending signals on purpose.  We call these technosignatures, and we discuss this in work done in September.  The other approach is by searching for molecules, like the recent Phosphine discussions, which would most likely indicate life. 

Currently, researchers around the world are working together in an attempt to build a list of molecules that would convincingly suggest life of some form is present.  Until this is better understood and we have this list, we can in the meantime look for the origin of molecules that are important to life here on Earth.  By studying their origin and the prevalence of them, we can start to understand the chances of them existing on other planets for the seeds of life.

In space, there are lots of free electrons running around and the gas tends to be at least partially ionised.  Exposure to the ionised gas creates free radicals, highly reactive molecules around stars and in the molecular gas of space.  

Two of the most studied free radicals are CH (Methylidyne) and OH (Hydroxyl).  The study of CH in objects outside of our own planet dates back to the 1940s when Belgian astronomers identified it in the optical spectra of the Cunningham 1940c comet.  Since then, it has been discovered and studied in many environments within our Galaxy and has been important to developing our understanding of the gas layers around stars. 

This simple molecule is important as carbon is a key component of life on Earth and there are a lot of carbon atoms floating around our Galaxy.  The reactive CH molecules tell an important story around the layers of gas surrounding the star along with the evolution of chemistry.

In our recent paper published in the Publications of the Astronomical Society of Australia, we completed the first search of CH at radio wavelengths (724 MHz) using an interferometer.  This is important as many assumptions about CH gas can be made when a single dish, such as how much the source fills the telescope beam and the density of the gas.

However, when an interferometer (a group of telescope dishes combined together) is used, it provides more precision on the location of the emission.  Curiously, we were able to detect the CH molecule with the CSIRO's Parkes 64m radio telescope (a single dish, also known by its traditional Wiradjuri name - Murriyang) but we didn’t find it with their Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope (a 36-dish antenna interferometer) when looking toward a region where new stars are being formed (HII region). This has left us pondering why this may have happened, despite having enough sensitivity that we should have seen it.  So far, we don’t have an answer.

The hydroxyl (OH) radical was first found in space by a radio telescope in the early 1960s, but at first, scientists didn’t know that is what they had found.  Originally it was called “Mysterium” before it was identified through modelling of the light the OH molecule emits at different frequencies.  

In atmospheres like that of Earth, OH is formed from the interaction of excited oxygen atoms with water.  The simple OH radical plays an important role in our atmosphere by neutralising pollutants and bacteria in our troposphere. In our own bodies, we produce OH and whilst it can also be toxic to our system, it’s also beneficial, as we’ve developed defences to deal with its presence.  

In space, OH is found in regions around stars through their whole life cycle and is linked to water production in molecular clouds and, when a star gets older and more unstable, the emission is boosted due to shock waves of energy from the star.  Therefore, there have been and will be in the future, large surveys to find more regions where OH can be found and what it can tell us about the life cycle of stars.

A lesser-known sibling of OH and CH is the radical NO (nitric oxide). Nitrogen and oxygen are two of the most common elements in our Galaxy (after Hydrogen, Helium, and Carbon), but for some reason, we don’t find the NO radical very often.  

This radical is of interest as a biosignature because we know it was important in the development of life on Earth, but also it is very important to our own biological systems.  If you search the world wide web for nitric oxide, you may find many supplements of the NO molecule to increase the efficiency of blood moving through your body and other natural processes that require it.

Although we have detected the NO molecule around evolved stars, dark molecular clouds, the interstellar medium, and star-forming regions, we still haven’t found it as often and as high of concentrations as we would expect.  So what does this mean?  One reason may be that we are not looking at the right part of the electromagnetic spectrum.  Most of the discoveries have been found in the region of 100’s of GHz to THz frequencies, what happens if we search at lower radio frequencies?

In our recent work published with Astrophysical Journals, we determined the precise frequencies which NO may emit light at the lower part of the radio spectrum.  We also then searched toward the Vela constellation for the signature of NO using the Murchison Widefield Array (MWA) telescope at 114 MHz.  

We did not detect NO in our survey, but that was not a surprising result as we will need much more time and improved data processing procedures to detect it.  We may have detected in our surveys toward the Orion Nebula and Galactic Centre at 107 MHz.  What it did tell us, however, is how big of a challenge searching at low radio frequencies for biomolecules is going to be.

Armed with the information of where we might find Nitric Oxide, we are going to start searching for it in targeted surveys using various Australian telescopes.  In particular, the ultra-wideband receiver on the Parkes 64m telescope will allow us to survey a wide range of radio frequencies in amazing detail to add to the search.

We are not stopping there.  We also hope to work with computational chemists, spectroscopists, and physicists to find at what frequencies other biomolecules can be found.  With this information, we can also determine how sensitive we would need our data to be.  All of this information combined can help inform the search for the Cradle of Life with the future low-frequency Square Kilometre Array, being built here in Western Australia. 
 

Both the Parkes radio telescope and ASKAP are owned and operated by Australia’s national science agency, CSIRO. We acknowledge the Wiradjuri people as the traditional landowners of the Parkes observatory and the Wajarri Yamatji as the traditional owners of the CSIRO's Murchison Radio-astronomy Observatory, which hosts ASKAP.