Category Science

In 1990, the Galileo space probe began to investigate Earth. It was determining whether it is possible to detect signs of life on a planet when viewed from space. The probe detected that Earth had water on its surface and oxygen in its atmosphere, which told scientists that the planet contained life. As well as this, Earth at night is like a glowing neon signpost, alerting aliens to our whereabouts. The Pioneer space probes had special plates engraved with symbols in case the probes ever encountered intelligent life on their journeys into space.

On December 8, 1990, the spacecraft Galileo swept by the Earth, picking up a gravity assist for its long journey to Jupiter. As it came within 960 kilometers of its home planet Galileo was wide awake, all its instruments active, taking measurements and collecting data. For the first time Earth was viewed and measured just like any other planet in the solar system, from the perspective of a spacecraft fly-by.

What could a spacecraft learn about our planet in such a brief visit? Would it detect the things we consider most significant about it – its rich vegetation, teaming life, and human presence? When in 1993 Carl Sagan, co-founder of The Planetary Society, published an article with colleagues in Science magazine about Galileo’s Earth encounter, these were precisely the questions he was trying to address. To their satisfaction, Sagan and his collaborators found that the evidence for the presence of life, and even intelligent life, was plentiful in Galileo’s data. No probe passing by Earth would miss the fact that here was a planet worthy of a more sustained investigation.

The signs of Earthly life picked up by Galileo were numerous: the combination of abundant water on the surface with unusual amounts of oxygen, methane and ozone in the atmosphere, radio signals, and more. One particular indicator that caught Sagan’s attention was the strong reflection of the near-infrared color in the Earth’s spectrum. This “red edge,” he wrote, pointed to the presence of “a light-harvesting pigment in a photosynthetic system.” It was, simply put, the signature of the green plant life that covers vast swathes of Earth’s surface, and it was strongest in those regions of the Earth that are covered with dense vegetation. A probe like Galileo, flying by an unknown planet, would be sure to detect this unmistakable indicator of rich plant life.

The time will probably come when; Star Trek-like, manmade probes will swoop by distant planets and collect data just as Galileo did for Earth. In the present, however, with our limited means and technology, no such close-range encounters are possible. For the foreseeable future, if life-bearing planets are to be found anywhere in the galaxy, they will have to be detected from distances measured by light years, not kilometers. Under such conditions, will the spectral indicator proposed by Sagan and his colleagues be of any use? Will we, in other words, be able to detect that “red edge” in the spectrum of a distant planet, and deduce from it the presence of plant life?

The question is of enormous interest to contemporary astronomers and planetary scientists. So far, admittedly, no Earth-sized planets have as yet been discovered, not to mention imaged, outside our solar system. But the day is undoubtedly near when the sensitivity of the extrasolar planet search will increase to the point when such planets will be found. A new generation of space missions, including the Space Interferometry Mission (SIM), the Terrestrial Planet Finder (TPF), and Darwin, are designed specifically for that purpose – to detect and observe Earth-like extrasolar planets. Within the coming years scientists will be in possession of spectrum measurements, and perhaps images, of distant planets with a mass and orbit comparable to Earth.

When this data becomes available, scientists will immediately begin mining it for signs of distant life. The most obvious and easy to detect signature of life, explains astronomer Pilar Montanes-Rodriguez of the Big Bear Solar Observatory (BBSO) in California, is the presence of large amounts of gases in a planet’s atmosphere than can coexist together only because life is sustaining them. Oxygen and methane, or oxygen, water, ozone and carbon dioxide, are examples of such combinations.

But this in itself only points to the presence of simple microbial life, such as existed on Earth for billions of years before the emergence of multicellular organisms. Detecting complex life, such as plants, is much more difficult, she explains, and for that scientists will need to rely on subtle indicators such as the “red edge.” How should we look for these signs of complex life on distant planets, and how likely are we to find them? These are the questions that Montanes-Rodriguez and her BBSO colleague, Enric Palle, set out to answer.

As well as receiving signals from outer space, radio telescopes such as Arecibo can also broadcast signals to the entire galaxy and beyond. In 1974, radio waves beamed from the Arecibo telescope carried a message deep into space. The message consisted of 1679 pulses that, when arranged into a grid 23 columns wide and 73 rows tall. The message was aimed at a dense ball of stars called M 13, which is so far away from Earth that it could take up to 50,000 years for a possible reply.

Serendip uses the world’s largest radio telescope to scan a fair fraction of the celestial sphere. This means it samples many billions of Milky Way stars and many thousands of background galaxies. No one star gets as deep a scrutiny as Project Phoenix provided, but the number of stars being scanned is immense.

No real-time followup yet. This is a problem for piggyback SETI, partly because weak signals from beyond several hundred light-years should fade in and out of audibility, on a timescale of minutes, due to “interstellar scintillation” caused by the thin gas between the stars. Therefore several repeat observations of each point on the sky will probably be needed to catch a single repeat of a continuous weak signal. And, of course, if the aliens turn their transmitter elsewhere (or off) before a dedicated follow-up is scheduled, the chance to confirm a signal disappears.

Astronomers have detected nearly 2,000 alien planets to date. As that number continues to rise, so too does the prospect of finding intelligent extraterrestrial life. In terms of the search for extraterrestrial intelligence (SETI), it may no longer be a matter of answering the “are we alone” question, some scientists say. Rather, just how crowded is the universe?

And if ET is out there, it may be possible to reach out with direct “radio waving” to potentially habitable exoplanets. This form of cosmic cryptography, called “Active SETI,” involves no longer merely listening for a signal but purposefully broadcasting to, and perhaps establishing contact with, other starfolk.

Active SETI sounds like science fiction, but some astronomers are discussing it seriously today. The idea is, as it has been in the past, a controversial, hot-button issue, with some researchers wary of sending signals out to touch base with intelligent aliens.

Radio astronomy is the most effective way to search for alien life. Radio telescopes can be positioned all over the world — as radio waves are not affected by the Earth’s atmosphere, and can pick up signals from across the Universe. Radio telescopes such as Arecibo in Puerto Rico, and the Very Large Array, enable astronomers to view space in all directions for signs of alien intelligence.

Several large searches for extraterrestrial intelligence (SETI) are currently scanning the stars, looking for both radio and laser transmissions from distant civilizations. Either type of signal could be sent across interstellar distances fairly economically, scientists are convinced.

Radio searches have been going on the longest. Most of them follow the same basic strategy: they hunt through the microwave part of the spectrum for any extremely narrowband (single-frequency) signal coming from outside the solar system. According to conventional wisdom, this is the kind of broadcast that has the best chance of being detected across interstellar distances.

Of the entire radio spectrum, the band of frequencies from about 0.5 to 60 gigahertz has the least natural background interference in space. Any alien radio astronomers should realize this too — and perhaps they would build interstellar transmitters accordingly. Our atmosphere generally limits us to frequencies below about 12 gigahertz, but maybe other civilizations would have reason to choose the low end of the frequency range too.

The only kind of transmission that we have much hope of detecting is a “beacon” — a very strong signal that aliens somewhere have deliberately designed to announce “Here we are!” as clearly and loudly as possible to any listeners in the cosmos, such as us. The searches now under way are much too weak to pick up any plausible radio chatter from another civilization’s internal traffic — its own broadcasts and point-to-point communications — no matter how advanced the civilization may be. (Indeed, there’s every reason to think that internal communications will become less recognizable from a distance as a civilization advances, judging from trends in our own communications technology.)

Considering the huge size of our galaxy, the immense distances between stars, and the immense width of the microwave radio spectrum, it’s a daunting task even to search for powerful beacons that are designed to help us out! SETI projects have advanced far in recent years, but we are still looking for needles in very big haystacks that remain almost completely unexplored.

Here is a complete rundown of all the major SETI efforts worldwide, both radio and optical, those have recently been carried out or are currently under way.

The astronomer frank drake pioneered the search for intelligent life elsewhere in the Universe. He claimed that for intelligent life, capable of communicating over inter-stellar distances, to arise on a planet, conditions must be perfect. He came up with an equation to estimate the number of civilizations in the galaxy with the means of communicating with Earth:

It might seem logical to assume that the first confirmed alien life forms will be microscopic bacteria hiding out in damp Martian soils or simple organisms swimming around the hidden seas of Europa. But a leading SETI scientist says it’s more likely we find evidence of extra-terrestrial intelligence before discovering alien bugs.

“There are two horses in the race to find life beyond Earth,” Andrew Siemion of the University of California, Berkeley, told the Association for the Advancement of Science conference in Seattle, according to News. “The first is the search for chemical signatures from planets and the second is the search for extra-terrestrial intelligence. Intelligent life has the edge, as it can be detected across the entire galaxy.”

Siemion gave a talk entitled “Hunting for Techno signatures” at the conference in the same week he announced a major data release from the Breakthrough Listen initiative, where he is principal investigator. (The data has yet to return any evidence of ETI.) Siemion explained that the search for simpler forms of life that we might intuitively assume are more abundant and therefore easier to find is actually quite limited. The problem is that technology available now and in the near future restricts us to looking at our solar system and nearby stars. “And we can never be sure that methane or similar chemicals which we detect are really produced by living things,” Siemion said. “It just comes down to statistics – basic life may be very common, but we are much less likely to find it.”

Intelligent life, on the other hand, can send evidence of its existence across the cosmos at the speed of light as radio waves, laser pulses or other forms of electromagnetic radiation. SETI scientists increasing refer to looking for these signals that could never be created by nature and other signs of alien technology as the search for technosignatures.

Now, as we race towards 2021 without seeing that detection, x is therefore becoming ever closer to zero. And 1,000 times zero equals zero. But Siemion confirmed that he is actually optimistic and he does believe in the other side of the equation, which indicates that our capacity to detect alien life will be three orders of magnitude greater in the coming decade than in the 2010s. “We’ve seen a dramatic explosion in the number of observatories, the number of scientists… that are working in this field,” he explained.

But if we do find simple life in our solar system before detecting ETI’s signals, it could point to a rather dark conjecture: that life in the universe is plentiful, but it rarely survives long enough to develop the capability to reach beyond its own world.

In May 2002, NASA chose two TPF mission architecture concepts for further study and technology development. Each would use a different means to achieve the same goal—to block the light from a parent star in order to see its much smaller, dimmer planets. The technological challenge of imaging planets near their much brighter star has been likened to finding a firefly near the beam of a distant searchlight. Additional goals of the mission would include the characterization of the surfaces and atmospheres of newfound planets, and looking for the chemical signatures of life.

Purpose: To search for Earth-like planets that might harbor life. Terrestrial Planet Finder will use multiple telescopes working together to take family portraits of stars and their orbiting planets and determine which planets may have the right chemistry to sustain life.

The mission will study all aspects of planets, from their formation as disks of dust and gas around newly forming stars to their subsequent development. It will also look for planets orbiting the nearest stars and study their suitability as homes for any possible life.

One great challenge is how to detect planets against the blinding glare of their parent star, an effort that has been compared to trying to find a firefly in the glare of a searchlight. Terrestrial Planet Finder will reduce the glare of parent stars to see planetary systems up to 50 light-years away. Using spectroscopic instruments on Terrestrial Planet Finder, scientists will measure relative amounts of gases like carbon dioxide, water vapor, ozone and methane. This study will help determine whether a planet is suitable for life–or even whether life already exists there.

NASA has chosen two mission architecture concepts for further study and technology development. The two candidate architectures are: Multiple small telescopes on a fixed structure or on separated spacecraft flying in precision formation would simulate a much larger, very powerful telescope. A technique called nulling would reduce starlight by a factor of one million, enabling the detection of the very dim infrared emission from planets.

Visible Light Coronagraph: A large optical telescope, with a mirror three to four times bigger and at least 10 times more precise than the Hubble Space Telescope, would collect starlight and the very dim light reflected from planets. Its special optics would reduce starlight by a factor of one billion, enabling astronomers to detect the faint planets.

Four hundred years ago, an astronomer named Giordano Bruno was burned at the stake for suggesting the existence of other Earth-like worlds. Today we know that there are potentially billions of extra solar planets in the Milky Way. None found so far resemble Earth. Indeed, many are shockingly different from our world. Although none of the planets investigated so far have shown any signs of life, many astronomers believe that it is only a matter of time before Earth’s twin planet is discovered.

Our solar system is just one specific planetary system—a star with planets orbiting around it. Our planetary system is the only one officially called “solar system,” but astronomers have discovered more than 2,500 other stars with planets orbiting them in our galaxy. That’s just how many we’ve found so far. There are likely to be many more planetary systems out there waiting to be discovered! Our Sun is just one of about 200 billion stars in our galaxy. That gives scientists plenty of places to hunt for exoplanets, or planets outside our solar system. But our capabilities have only recently progressed to the point where astronomers can actually find such planets.

Even our closest neighboring stars are trillions of miles away. And all stars are enormous and extremely bright compared to any planets circling them. That means that picking out a planet near a distant star is like spotting a firefly right next to brilliant lighthouse miles away.

So far, the planets outside our solar system have proven to be fascinating and diverse. One planet, known as HD 40307g, is a “super Earth,” with a mass about eight times that of Earth. The force of gravity there would be much stronger than here at home. You would weigh twice as much there as you do on Earth! Another planet, called Kepler-16b, turns out to orbit two stars. A sunset there would provide a view of two setting stars!

In another planetary system, called TRAPPIST-1, there are not one…not two…but seven Earth-sized planets that could be covered in liquid water. The planets are relatively close together, too. If you were to stand on the surface of a TRAPPIST-1 planet, you might see six other planets on the horizon!