Of the over 150,000 stars monitored by the Kepler spacecraft for evidence of exoplanets, or planets outside of our solar system, the star KIC 8462852 may be the most mysterious.
The task of Kepler, launched in 2009, is to watch for a slight dimming of the light from each star, caused by planets transiting across the face of the star. The light curve created by a transiting planet usually registers a one or two percent dimming of the star’s light, and is precisely regular and predictable.
In 2011, highly irregular light curves were detected from KIC 8462852, as reported in The Atlantic (October 13, 2015), with the starlight dimming by as much as an unprecedented 22 percent. The strange star was first noticed by “citizen scientists” working with the Yale-based Planet Hunters group, who brought the data to the attention astronomer Tabetha Boyajian and her fellow astronomers at Yale. After ruling out faulty data, the astronomers tried to find a natural explanation for the extreme dimming. One possible, if unlikely, explanation was suggested: perhaps a nearby star had destabilized comets orbiting KIC 8462852, causing the comets to move closer to the star, blocking its light from view.
But, a more exotic explanation for the star’s dimming was proposed by Penn State astronomer Jason Wright: maybe the diminished starlight was evidence for an alien megastructure, such as enormous solar collectors orbiting the star. “Aliens should always be the very last hypothesis you consider, but this looked like something you would expect an alien civilization to build,” said Wright in The Atlantic article.
Almost immediately after the article was published, the “alien megastructure” explanation caught fire on the internet. Harvard astrophysicist Dimitar Sasselov, who is teaching an EdX online course, “Super-Earths and Life,” with an enrollment of over 10,000 students, says that as soon as the article appeared, “my students were already talking about it, and when you have 10,000 students talking about it, a lot happens. Even before I jumped in…some of the students had already gone to the NASA database, pulled out the light curve and the data from Kepler and put it on our website.”
Sasselov says the excited response by his students points to a fascination about the possibility of discovering extraterrestrial life. “This is a very unusual and special time in the history of science, where the question that has been asked for centuries is really tractable by our methods and technology.”
And, if you ask NASA, the question of whether we are alone in the universe will soon be answered.
“I think we’re going to have strong indications of life beyond Earth within a decade, and I think we’re going to have definitive evidence within 20 to 30 years,” NASA chief scientist Ellen Stofan said during an April 7, 2015 panel discussion, reported by CNN.com, which focused on NASA’s search for habitable planets and alien life. “We are not talking about little green men,” said Stofan. “We are talking about little microbes.”
Former astronaut John Grunsfeld, who is now Associate Administrator for NASA’s Science Mission Directorate, also predicted near-term discoveries of alien life during the same panel discussion. “I think we’re one generation away in our solar system, whether it’s on an icy moon or on Mars, and one generation [away] on a planet around a nearby star.” A video of the panel discussion is available here.
Exoplanets Beyond Measure
The NASA officials’ comments come amid the ongoing discovery of new planets orbiting distant stars. And among the 1,903 confirmed exoplanets, a small subset of these planets are believed to be compatible with the development of life. These are earth-sized rocky planets with significant amounts of water, which revolve around their host star in the habitable or “goldilocks” zone: not too close to the heat of their star, so that the water turns to steam, and not too distant, so that the water freezes.
The total number of habitable-zone exoplanets is believed to be almost immeasurable. The deep field images from the Hubble Space Telescope have allowed astronomers to roughly estimate the number of galaxies in the observable universe to between 100 to 200 billion. Within the earth’s Milky Way galaxy alone, there may be 40 billion earth-sized planets in habitable zones, according to the October 22, 2013 National Academy of Sciences report.
The explosion in exoplanet detection is made possible by new telescope technology and by new search methods. By far, the most productive technique has been the transit method, utilized by the Kepler spacecraft, which detects a slight dimming of a star as a planet transits across the face of the star. As of January 6, 2015, 1,004 exoplanets had been confirmed by Kepler, with another 3,200 planet candidates waiting to be confirmed, according to a report at Space.com.
A number of additional instruments will be added to the astronomer’s arsenal over the next decade (see text box below), which will aid in the search for extraterrestrial life. The James Webb Space Telescope and the three large ground-based telescopes will allow for the direct imaging of larger exoplanets and for the analysis of exoplanet atmospheres utilizing a spectrograph. As starlight shines through the atmosphere of an exoplanet, “you see these atomic finger prints, and so you can tell what atomic or molecular components make up that” atmosphere, said Harvard astrophysicist Andrew Szentgyorgyi. Gases found in exoplanet atmospheres may be biosignatures, that is, gases that indicate the presence of life, or life as we know it, and which cannot be present in the absence of life, such as persistent low levels of methane.
|Future Tools in the Search for Alien Life
A number of new, powerful instruments will be available to astronomers in the near future, as they search for evidence of extraterrestrial life:
Set to launch in 2017, the TESS will be an all-sky satellite, surveying small, close stars using the transit method for earth-like planets.
James Webb Space Telescope (JWST)
Set to launch in 2018, the infrared JWST will have image resolution seven times better than the Hubble, analyzing the atmospheres of exoplanets for gases that co-exist with life.
Three very large ground-based optical telescopes are scheduled to become operational in the next decade. All of these telescopes utilize adaptive optics, which correct for the aberrations of the atmosphere, making very sharp images possible.
Giant Magellan Telescope (GMT)
Set to begin operation in 2020, the GMT is being built on a Chilean mountain top, and will have image resolution ten times better than the Hubble.
Set to begin operation in 2024, the E-ELT is also being built on a Chilean mountain top, and will be the largest optical telescope in the world.
Thirty Meter Telescope (TMT)
Set to begin operation in 2024, the TMT is being built on top of a mountain in Mauna Kea, Hawaii. Construction is currently halted, due to protests from indigenous Hawaiians, who consider the mountain to be sacred ground.
Spectral analysis of exoplanet atmospheres will be a key strategy in the search for alien life. However, there are other natural processes which result in elevated levels of those same gases. “For a long time we will rely upon spectrum measurements of distant atmospheres. For example, when free oxygen co-exists with methane, as it does on Earth, it suggests, but does not prove, the existence of life,” said David Brin, astrophysicist and science writer. Brin mentioned another limitation of atmospheric spectral analysis for the detection of life: “What about ice-roofed worlds, like Europa? These may be the majority of zones containing living environments, and they emit very little [atmospheric gases] that could be detected at long range.”
MIT astrophysicist Sara Seager, who intends to devote her career to the search for extraterrestrial life, concedes, “It’s definitely bothersome that many of the gases that life produces are also produced by other means: geophysics, volcanoes, atmospheric processes…Unfortunately, we will never definitively be able to say if there is life or not. But we could find very strong evidence for life,” said Seager. “Am I happy with that? No. Do I have a choice? No.”
Johns Hopkins biologist Jocelyne DiRuggiero, who is an expert on extremophiles, organisms that thrive in extreme environments, concurs with Seager: “Outside of our solar system it will be very difficult to go beyond evidence – and probably not definitive at that – for a long, long time…Unless we can land on one of these planets, and take measurements, we will never be sure.”
Chris McKay, planetary scientist at the NASA Ames Research Center, who has published extensively on planetary atmospheres and the evolution of life, is optimistic life will be found throughout the universe. He cites four reasons:
- “Life appeared on Earth very early in its history, 3.5 billion years ago and arguably 3.8 billion years ago;
2. Organic compounds, including amino acids, form naturally in many environments in space. They are seen in meteorites, comets…the natural chemistry of carbon in the universe is organic;
3. There are clearly lots of Earth-like exoplanets that could have water on them;
4. The essential elements that life (as we know it) needs (H2O, C, N, P, S) appear to be common everywhere.”
But, McKay takes issue with NASA’s Ellen Stofan, who predicts “definitive evidence” for life beyond earth in the near future. “We can’t say we are going to have evidence of life. We just don’t know if it’s there. We hope it is, we expect it to be, but Nature is not bound by our hopes and expectations.”
Life Within Our Solar System
Space scientists continue to search for life on planets and their moons within our own solar system, utilizing space probes. There have a few tantalizing findings, such as the presence of methane spikes on the Mars surface, detected by the Curiosity rover, which could be the result of either an active geological process or of the presence of biology.
In September, NASA announced that its Mars Reconnaissance Orbiter had detected evidence for flowing water on Mars. But McKay warns that the news “is mostly hype. The ‘liquid water’ discovered on Mars is brine…Such a brine is not suitable for life and is of no interest for biology. The result is of interest only geologically.”
“I am not optimistic that there is sub-surface microbial life on Mars,” said McKay. “In my view our best chances on Mars are to find life that is long dead and preserved in a frozen state.” He believes the best place to look for life in this solar system in on Saturn’s moon Enceladus, which spews forth plumes of water from a warm sub-surface ocean. Future probes could fly through these plumes and analyze the water for signs of life. The NASA Enceladus Life Finder (ELF) mission is in the proposal stage, and, if approved, would launch in 2021. As McKay told the New York Times, “My mantra now is follow the plume.”
On October 28, the Cassini spacecraft, which has been orbiting Saturn since 2004, passed through one of the plumes of Enceladus, flying only 30 miles above the moon’s surface. Although Cassini was not designed to analyze samples for the presence of life, previous higher passes by Cassini through the plumes have detected molecules associated with life, including water vapor, methane, and formaldehyde, as reported in the October 28 New York Times.
Harvard astrophysicist Sasselov says that, because Enceladus is so small, only about 300 miles in diameter, “I would put Enceladus on the bottom of my list.” He would rather focus on Europa, Jupiter’s ice-covered moon, believed to have a large water ocean below the ice, or on Titan, Saturn’s cloudy moon with stable methane lakes on its surface. But a mission to Europa would be “tough. It requires making a hole through the ice crust…who knows how deep we’d have to go,” said Sasselov. “Titan, on the other hand, has the liquid lakes on the surface. We are talking about a completely alien world…I think the chemistry [of the lakes] is fascinating, and who knows…”
Alien Signals, Alien Threats
For over 40 years, astronomers have passively searched the skies with radio telescopes, hoping to detect signals from intelligent civilizations in space. Thus far, the Search for Extraterrestrial Intelligence (SETI) project has been unsuccessful. But the search has been very small and has very far to go. “Their search so far has been equivalent to one glass of water compared to all of the world’s oceans,” said Seager.
Physicist Stephen Hawking recently announced his support for the $100 million Breakthrough Listen, the largest radio telescope search ever undertaken to detect alien intelligence. “It’s time to search for life beyond Earth. The Breakthrough initiatives are making that commitment. We are alive. We are intelligent. We must know,” said Hawking at the Breakthrough news conference on July 20, 2015, as reported by Space.com.
Since 2010, Hawking has been warning against humanity actively sending messages into space in an attempt to communicate with alien intelligences. “A civilization reading one of our messages could be billions of years ahead of us. If so, they will be vastly more powerful, and may not see us as any more valuable than we see bacteria.”
But Sasselov is “not concerned about this because it’s too late. Our radio and TV signals are already way out in the space of the galaxy. Whether we send signals or we don’t send signals, we can’t hide.”
Aliens Among Us
Obviously, if aliens land on the White House lawn, the question of extraterrestrial life would be instantly answered. On the topic of UFO’s, MIT’s Seager says she is “a definite nonbeliever… If there are so many UFO’s visiting us all the time, surely some of them would leave some evidence, a scrap of real evidence that could be tested.” Seager also doubts the ability of intelligent aliens to travel to the Earth. “I don’t believe that it’s easy to travel these large distances. I don’t believe that one can travel close to the speed of light.” She notes that “a large part of the exoplanet fan base are UFO believers… a more interesting question is why so many people want to believe.”
NASA’s McKay says of UFO’s, “I am not opposed, in principle, to alien artifacts on the Moon and Mars, or alien visitations…but the standard of proof must be high. To reuse the saying popularized by Carl Sagan ‘extraordinary claims require extraordinary evidence’.”
Harvard’s Sasselov believes that, if we are visited by aliens, the beings will likely be machines. “Biological entities are just not appropriate for the scale or the radiation environment of the galaxy,” he said. Rather, the aliens will be “some kind of non-biological entities that have sentience, but are much more efficient energetically…Any visitation will be something we won’t even recognize as life.”
Harvard astrophysicist Andrew Szentgyorgyi speculates that the discovery of life beyond earth “will be evolutionary rather than revolutionary…There are all of these different things people are trying to do to find extraterrestrial life, and one might just work out, all of a sudden, bam, there’s just no question about it.” But, he rather suspects that “it’ll be a steady accumulation of different data that will slowly coalesce on an opinion that we have found exobiota.”
A scientific opinion that life has probably been found elsewhere in the universe, based on the preponderance of the evidence, but without absolute certainty, will satisfy no one. But this may be all that astronomers and the public can expect or hope for, for a very long time.