There is no evidence of life beyond Earth. However, scientists use transit spectroscopy, gravity microlensing, and coronagraph to detect gases that may indicate the presence of life on planets outside our solar system.
In the 19th century, astronomers believed other planets were likely inhabited. Early space exploration changed this view. Today, only Earth is known to host life. However, scientists believe life might be common across many worlds.
Yes, scientists believe that life is likely common across many worlds. Evidence from astrobiology suggests that simple life, made up of individual cells or small multicellular organisms, is common throughout the universe. This life may have occurred multiple times in our own solar system.
The panspermia hypothesis is a philosophical idea that life can naturally migrate through space. This hypothesis states that the seeds of life exist throughout the universe and can be spread from one location to another.
NASA scientists are looking for signs of intelligent life, which they call technosignatures, among the thousands of exoplanets confirmed in the galaxy. Technosignatures could include:
- Signals from radio or optical light waves
- Synthetic gases in an exoplanet’s atmosphere
The search for life beyond Earth is still in its early stages. The universe’s billions of years of existence would allow plenty of time for intelligent lifeforms to traverse the galaxy, even at slow travel speeds.
According to HowStuffWorks, scientists estimate that there are 60 billion planets in the Milky Way that could support life. Considering the vast number of galaxies, researchers estimate about 50 sextillion potentially habitable planets in the universe
However, the Drake equation estimates that only one in a million million planets has the right combination of chemicals, temperature, water, days and nights to support life as we know it. This calculation arrives at the estimated figure of 100 million worlds where life has been forged by evolution.
As of June 2021, a total of 59 potentially habitable exoplanets have been found
Scientists estimate there could be 60 billionplanets in the Milky Way alone within habitable zones capable of supporting life. Considering the vast number of galaxies, researchers estimate about 50 sextillion potentially habitable planets in the universe, making Earth one of many candidates for hosting life
According to the European Space Agency, the presence of ozone, liquid water, and carbon dioxide simultaneously is a strong indicator of life. Other signs of life on a planet include:
- Carbon-lite atmosphere A low carbon abundance in planetary atmospheres could be a signature of habitability.
- Spectroscopy This technique analyzes light shot by a star through the atmosphere of a distant planet, and the effect looks like a bar code. The slices missing from the light spectrum tell us which chemicals or gases are present in the alien atmosphere. For example, one pattern of black gaps might indicate methane, another, oxygen.
- Chemical traces Life on Earth leaves traces in the chemical makeup of the atmosphere, which are visible from a long way away. For example, scientists have reported chemical traces in the atmosphere of a planet called K2-18b, which is about 124 light-years from Earth. In particular, they may have detected a substance which
Fridlund says, “The general consensus is that if we find ozone, liquid water, and carbon dioxide simultaneously, it will be a very strong indicator of life’s presence.”
If we could detect a clear, unambiguous biosignature on just one of the thousands of exoplanets we know of, it would be a huge, game-changing moment for humanity. But it’s extremely difficult. We simply aren’t in a place where we can be certain that what we’re detecting means what we think or even hope it does
A fundamental goal of astrobiology is to detect life outside of Earth,” write the authors of a new paper. It’s titled “An Agnostic Biosignature Based on Modeling Panspermia and Terraformation,” and it’s available on the pre-press site arxiv.org. The authors are Harrison B. Smith and Lana Sinapayen. Smith is from the Earth-Life Science Institute at the Tokyo Institute of Technology in Japan, and Sinapayen is from the Sony Computer Science Laboratories in Kyoto, Japan
Scientists are pretty good at modelling things and trying to generate useful answers, as well as generating relevant questions they might not have thought of without models. In this work, the pair of authors took a different approach to understanding life on other worlds and what effort we can make to detect it.
“Here we explore a model of life spreading between planetary systems via panspermia and terraformation,” the authors write. “Our model shows that as life propagates across the galaxy, correlations emerge between planetary characteristics and location and can function as a population-scale agnostic biosignature.”
What it would take to detect that life, if it exists, isn’t a new question. But thanks to the JWST, it’s finally becoming a practical one. In the next few years, the telescope could glimpse the atmospheres of several promising planets orbiting distant stars. Hidden away in the chemistry of those atmospheres may be the first hints of life beyond our solar system. This presents a sticky problem: What qualifies as a true chemical signature of life?
“You’re trying to take very little information about a planet and make a conclusion that is potentially quite profound – changing our view of the whole universe,” says planetary scientist Joshua Krissansen-Totton of the University of Washington
Perhaps the most intuitive way to look for a biosignature in that barcode is to scour it for a gas that was clearly produced by life. For a time scientists thought that oxygen, which is abundant on Earth because of photosynthesis, served as a stand-alone biosignature. But oxygen can arise from other processes: Sunlight could break apart water in the planet’s atmosphere, for example.
And that problem isn’t unique to oxygen – most of the gases that living things produce can also arise without life. So instead of treating single gases as biosignatures in their own right, scientists today tend to consider them in context.
Surprises, not assumptions
Of course, “if you’re looking for individual gases like oxygen or methane, then built into that are assumptions about what type of life is elsewhere,” says Krissansen-Totton. So some scientists are developing agnostic biosignatures that don’t assume alien biochemistry will be anything like Earth’s biochemistry.
One possible agnostic biosignature is an exoplanet atmosphere’s degree of chemical “surprisingness” – what scientists call chemical disequilibrium
If we can observe enough rocky exoplanets, “we’re going to be in a much, much stronger place to understand what a biosignature means,” says Wordsworth. “One really powerful thing that exoplanets give us is statistics
We live in an age of remarkable exploration and discovery. Fully half of the nearby Sun-like stars have circumstellar disks of gas and dust like the solar nebula out of which our planets formed 4.6 billion years ago. By a most unexpected technique — radio timing residuals — we have discovered two Earth-like planets around the pulsar B1257+12. An apparent Jovian planet has been astrometrically detected around the star 51 Pegasi. A range of new Earth-based and space-borne techniques–including astrometry, spectrophotometry, radial velocity measurements, adaptive optics and interferometry– all seem to be on the verge of being able to detect Jovian- type planets, if they exist, around the nearest stars. At least one proposal (The FRESIP[Frequency of Earth Sized Inner Planets] Project, a spaceborne spectrophotometric system) holds the promise of detecting terrestrial planets more readily than Jovian ones. If there is not a sudden cutoff in support, we are likely entering a golden age in the study of the planets of other stars in the Milky Way galaxy.
Once you have found another planet of Earth-like mass, however, it of course does not follow that it is an Earth- like world. Consider Venus. But there are means by which, even from the vantage point of Earth, we can investigate this question. We can look for the spectral signature of enough water to be consistent with oceans. We can look for oxygen and ozone in the planet’s atmosphere. We can seek molecules like methane, in such wild thermodynamic disequilibrium with the oxygen that it can only be produced by life. (In fact, all of these tests for life were successfully performed by the Galileo spacecraft in its close approaches to Earth in 1990 and 1992 as it wended its way to Jupiter [Sagan et al., 1993].)
The best current estimates of the number and spacing of Earth-mass planets in newly forming planetary systems (as George Wetherill reported at the first international conference on circumstellar habitable zones [Doyle, 1995]) combined with the best current estimates of the long-term stability of oceans on a variety of planets (as James Kasting reported at that same meeting [Doyle, 1995]) suggest one to two blue worlds around every Sun-like star. Stars much more massive than the Sun are comparatively rare and age quickly. Stars comparatively less massive than the Sun are expected to have Earth-like planets, but the planets that are warm enough for life are probably tidally locked so that one side always faces the local sun. However, winds may redistribute heat from one hemisphere to another on such worlds, and there has been very little work on their potential habitability.
Nevertheless, the bulk of the current evidence suggests a vast number of planets distributed through the Milky Way with abundant liquid water stable over lifetimes of billions of years. Some will be suitable for life–our kind of carbon and water life–for billions of years less than Earth, some for billions of years more. And, of course, the Milky Way is one of an enormous number, perhaps a hundred billion, other galaxies.
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