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The breaking dawn over a distant sky / Michael D. Lemonique

The galaxy is teeming with planets. Scientists are trying their best to peer into their atmospheres to look for signs of extraterrestrial life

An alien world with a large moon. Illustration: shutterstock
An alien world with a large moon. Illustration: shutterstock

Anyone who was there during those hours, from the oldest astrophysicist to the most novice science reporter, will probably never forget the press conference held at the winter meeting of the American Astronomical Society in San Antonio, Texas in January 1996. At that place and at that time, Jeffrey W. Marcy announced, who was then working as an observer at San Francisco State University, that he and his observing colleague, R. Paul Butler, then at the University of California, Berkeley, discovered the second and third planets ever found orbiting our Sun-like star. The existence of the first planet of this type, 51 Pegasi b, was announced a few months before by Michel Mayor and Didier Klo from the University of Geneva. But a single identification may be accidental or even a mistake. Now Marcy could say with confidence that there was no accident and no mistake. "Planets," he told the crowd, "are not rare after all."

This announcement shook the world of astronomy. Almost no one looked for planets outside our solar system because scientists were convinced that they would be too difficult to find. And now, after searching only a small handful of stars, astronomers have discovered three worlds, evidence of billions more waiting to be discovered.

If Butler and Marcy's discovery had amounted to settling a question in the theory of planet formation, it would not have been such an important achievement. However, this discovery showed unequivocally that planets outside the solar system do exist, and together with them there is also the possibility to answer a question that has troubled the rest of philosophers, scientists and theologians since the days of ancient Greece: are we alone in the universe?

After the initial cheers died down, scientists sat down to figure out how exactly they were going to investigate the possibility of life, even the most primitive, on the surface of the holiday planet around an alien sun.

Other than picking up an extraterrestrial radio transmission, Jodie Foster-style in the movie Contact, the only way to find out is to look for atmospheric biological signatures on these planets, that is, evidence of highly active molecules, such as oxygen, that would have disappeared quickly from the atmosphere, unless there were organisms of some kind. Restock through metabolism.

The observations of Marcy, Maior and their colleagues were based solely on the gravitational influence of the planets on their parent star. To discover a biological signature, it is necessary to "photograph" the exo-atmosphere, i.e. an extraterrestrial atmosphere, directly. To this end, NASA planned to launch a series of space telescopes whose power will gradually increase, a program that will culminate in a telescope that will circle the Earth, and its name is the Interferometer for finding Earth-like planets, the construction of which will cost billions of dollars and it should be launched sometime in the next decade. In short, astronomers knew that they were not going to learn anything about the atmospheres of exoplanets, that is, planets outside the solar system, in the foreseeable future.

Kepler space telescope. Image: NASA
Kepler space telescope. Illustration: NASA

they were wrong. Discoveries of these few planets inspired a whole generation of young scientists, spurring them into the new field that was at one time the most talked about specialty in astrophysics. Many of their older colleagues were also convinced as a result to move into the field of exoplanetology. This sudden influx of brains led to new ideas for the study of atmospheres outside the solar system and gave a tremendous impetus to the advancement of the field. In 2001, astronomers already discovered sodium in the atmosphere of one of these planets. In the time that has passed since then methane, carbon dioxide and water have also been identified. Scientists have even found indirect hints, by examining the atmospheres of planets outside the solar system, that some of them may be partly made of pure diamond. "At this point," says Heather Knutson, an astrophysicist from the California Institute of Technology ("Cal-Tech") who was involved in many of these groundbreaking observations, "we've learned something about the atmospheres of between 30 and 50 planets, if you also count Data not yet officially published."

These discoveries are still far from providing evidence for the existence of life, and this is not surprising, since most of the worlds Natson talks about are hot planets, similar to the planet Jupiter, which orbit their star in a tighter orbit than the orbit of our hot star (Mercury) around the Sun. However, Knutson and her colleagues began to look more and more towards the atmospheres of smaller planets, of the type known as "super-Earths", whose masses are two to ten times greater than the mass of our Earth - an observational feat that no one could even imagine Ten years ago. In April 2013, it was announced that the Kepler space telescope had discovered two planets with a mass less than twice the mass of Earth, both of which are in orbits where temperatures may allow life to exist. This announcement provided further evidence that there is almost no doubt that life-friendly worlds exist in abundance. Although these planets, named Kepler 62e and 62f, are too far away to be studied in detail, astronomers are convinced that it won't be many years before observers can look for biological signatures in the atmospheres of planets that could in principle be considered twin brothers to Earth.

The parking lot planet

Astronomers assumed that it would be decades before we could begin to observe planetary atmospheres because the handful of first exoplanets discovered outside of Earth were discovered indirectly, through the influence each had on its parent planet. The planets themselves were invisible, but since each planet and its parent star orbit a common center of gravity, the planet's gravitational pull makes the star appear to be wobbly in place. When a star moves towards us, its light is slightly deflected towards the blue side of the visible light spectrum and when it moves away from us, the light is deflected towards the red. From the deviation rate, observers can deduce the radial velocity of the star, that is, how fast it is moving towards the Earth or away from it, and from this it is possible to deduce the mass of the planet that surrounds it (and caused the oscillation).

However, astronomers had another option for finding planets. If the invisible orbit of the planet is at such an angle that observers from Earth are exactly on the side, the planet will pass exactly across its star, a phenomenon known as "astronomical transit". But in the days of those first discoveries, some twenty years ago, almost no astronomer thought about transits at all, simply because even the search for the planets themselves was so marginalized. (One notable exception is William J. Borocki of NASA's Ames Research Center, principal investigator on the Kepler space telescope mission, which will eventually find transiting celestial bodies by the thousands.)

A few years later, in 1999, Timothy W. Brown, then working at the US National Center for Atmospheric Research, and David Charbonneau, then an advanced graduate student at Harvard University, set up a tiny telescope the size of an amateur telescope in a parking lot in Boulder, Colorado and saw such a transition for the first time. The planet was HD 209458b, previously identified using the radial velocity method. A few weeks later, Gregory W. Henry of Tennessee State University, working with Marcy, observed the same planet transiting its star. The first right to the discovery was given to both teams together because both identifications were published at the same time.

The successful detection of these transits gave astronomers a second way to find planets outside the solar system, but more than that: it also gave them a way to measure their density. The radial velocity method revealed the mass of HD 209458b. Now astronomers also knew its physical size because the amount of starlight blocked by a planet is directly proportional to its size. (HD 209458b is 38% larger than Jupiter, although, in terms of mass, it is 29% lighter than Jupiter; dividing its mass by its size gives an unexpected consequence of its density, which astrophysicist Adam Burroughs of Princeton University called "an ongoing problem that requires explanation.")

At this stage, several astrophysicists have already realized that transits also make it possible to study the atmospheres of planets outside the solar system, in a process that Natson calls "an incredibly clever shortcut". In fact, even before a transition was first reported, Sarah Seeger, an astrophysicist at the Massachusetts Institute of Technology (MIT), who was then studying for an advanced degree with Charbonneau at Harvard, published a paper with her supervisor, Dimitar D. Saslov, in which they predicted what observers would see When light emitted from a star will pass through a planet's atmosphere while the planet is passing the star. Physicists have known for a long time that different molecules and atoms absorb light at different wavelengths. If you look at the planets at a wavelength that matches the molecule you are looking for, any atmosphere containing that molecule will absorb the light. The planet's airy atmosphere will become opaque, and the planet will appear larger.

Seeger and Saslov raised the possibility that sodium vapor would be particularly easy to detect. "Sodium is like the stench of a skunk," says Charbonneau. "Even a small portion will do the trick." And who knows: in 2001 Charbonneau, Brown and their colleagues returned to HD 209458b, the planet whose transit was the first to be measured, but this time not with a poor amateur telescope, but with the Hubble Space Telescope. Indeed, the sodium seal was also there, exactly as predicted.

Full eclipse

Astronomers have also realized that there is a second way, complementary to the first, to examine the atmospheres of transiting planets. When a planet transits its sun, it turns its night side to the observer on Earth. In other situations it shows at least part of its day side, and just before the planet is hidden behind the star, almost all of its day side faces Earth. Although the star is much brighter than the planet surrounding it, the planet itself also glows, mainly in infrared light.

But the glow disappeared at once as the planet moved behind the star; Its contribution to the common light of the planet and the star disappears. If the astrophysicists can make a comparison of "before" and "after", they will be able to deduce what the planet would look like if it were alone [see box on next page]. "This method changes the nature of the problem," says Knutson. "Instead of having to detect something very faint standing next to something very bright, all you have to do is measure signals that change over time." L. Drake Deming, who worked at NASA's Goddard Space Flight Center, already in 2001 aimed the infrared telescope on the Mauna Kea volcano in Hawaii at HD 209458b, trying to see this phenomenon, known as a secondary eclipse, but according to him he did not He managed to come to an identification.

However, he knew that the Spitzer Space Telescope, scheduled for launch in 2003, would almost certainly be able to make such an observation, and Sharvonno agreed with him. The two astronomers, who did not know each other, submitted a request to Spitzer to make the observations. Both received the time period they requested and collected the data. Then, one day in early 2005, Deming recalled, he got a voice message: "Drake, this is Dave Charbonneau from Harvard," the voice said. "I heard you made some interesting observations recently. Maybe we should talk."

It turned out that Deming (who worked with Seeger) and Sherbon, each separately made the first secondary eclipse detections in history, at the exact same time, using the same observatory. The two groups simultaneously published their results measured on two different planets: HD 209458b plowed to fatigue in Deming's case and a planet known as TrES-1 in Charbonneau's. A year later, Deming's team detected the secondary eclipse of a planet known as HD 189733b. "This discovery," wrote Seeger and Deming in a review article in 2010, "was followed by a flood of observations that identified secondary defects with the help of 'Spitzer'... It can be said unequivocally that no one foresaw the full power and the tremendous impact of the 'Spitzer Space Telescope' ' as a tool for developing the research field of atmospheres outside the solar system." In fact, says Seeger, "we're using Hubble and Spitzer in ways they were never intended to operate, and reaching measurements with precision many digits past the point they were never intended to reach."

atmospheric layers

These studies showed two things, Seeger says. "This may sound a bit like yesterday's news, but we have learned that so-called 'hot Jupiter-like' planets are indeed hot. We measured their luminosities and their temperatures,” and what came up in the observations matched how they expected stars to heat their planets. "Secondly, we identified molecules. Are [our findings] very different from what we expected? The truth - not really." Seeger says that physicists have paved ways that allow them to build a model of a ball of gas at a certain temperature, consisting of some combination of elements, and ask what types of molecules will be formed. "The laws of physics and chemistry are universal," she says.

However, Seeger and other astrophysicists also learned that despite the general similarity between the atmospheres of different planets, there are several ways to differentiate one planet from another. One way is the way the temperature changes with the rom (the height above the ground of the planet). There are planets, such as Jupiter and Saturn in our solar system, where you can see an inversion of temperatures, that is, a situation where the temperature rises with the moon instead of falling. In other planets the situation is different. "The problem," says Knutson, "is that we don't know what causes the inversion, so we can't predict which planets will have this feature and which won't." Some astrophysicists say that planets with inversions may have heat-absorbing molecules of some kind, such as titanium dioxide, but for now that's just speculation.

Another question is whether certain planetary atmospheres are made of a different mixture of molecules than the mixture on other planets. Niko Madhusudhan, now at Yale University, analyzed the visible and infrared light signature of the planet WASP-12b and concluded that its atmosphere was unusually rich in carbon: the proportion of this element in the atmosphere was similar to that of oxygen.

According to the theory, the carbon-oxygen ratio is higher than 0.8, if it also appears in other, smaller planets in the same solar system (as it is likely to happen, since the explanation is that planets in the solar system are formed in a condensation process from a single disk of gas and dust) , will lead to the creation of "rocks" made of carbides, minerals rich in carbon, instead of silicate rocks rich in silicon (silicon) found in our solar system. If this is indeed the case, an Earth-sized planet in the WASP-12b system may have continents made of diamonds.

Seeger and others have written theoretical papers raising the possibility that there is no reason to rule out the existence of planets made mostly of carbon or even iron. But in the case of WASP-12b, this may not be the case. Knutson says that Ian Crossfield of the Max Planck Institute for Astronomy in Heidelberg, Germany, recently discovered that the light from WASP-12b is contaminated by light from a fainter background double star. "His data may cast some doubt on the interpretation of this particular planet," Knutson says.

water World

The observation that has received the most attention, far more than any other observation, focuses on a planet known as GJ 1214b, orbiting a reddish "M-dwarf" star (an M-dwarf is a red dwarf star with spectral classification M), located about 40 light-years away. from the earth Thanks to the proximity of GJ 1214b to us, it is relatively easy to study it. And its diameter is only 2.7 times that of Earth, meaning the planet is much closer to being Earth-like than the hot Jupiter-like planets found in the early years of planet hunting. "This planet is everyone's favorite super-Earth," says Laura Kreidberg, a research student at the University of Chicago who is leading the data analysis for one such observational project.

The planet GJ 1214b was discovered in 2009 as part of the MEarth project organized by Charbonneau to search for holiday planets around Nancy-M. The idea behind the project is that it will be easier to find small transiting planets in the vicinity of those small, dim stars than in the vicinity of larger stars, for several reasons. First, a planet the size of Earth blocks very little light, so relatively it will block a much larger proportion of light coming from a smaller planet. Such a planet would also exert a relatively stronger gravitational pull on the star, so it would be easier to estimate the planet's mass and, in any case, its density. Also, the area suitable for life in the case of a small, cold star will be much closer to it than in the case of a hot, sun-like star, so it will be much more likely that we will be able to detect transits (because the orbit of a nearby planet can pass over the star even without being aligned precisely with our field of vision). And finally, there are many more M-dwarfs than Sun-like stars in the Milky Way: about 250 M-dwarfs lie within about 30 light-years from Earth, compared to about 20 Sun-like stars.

But GJ 1214b is not exactly a second Earth: its diameter is 2.7 times that of Earth and its mass is 6.5 times greater, so its overall density is between that of Earth and that of Neptune. Unfortunately, as Sherbono and others realized soon after the star was discovered, this density can manifest itself in several different ways. It could, for example, have a small rocky core surrounded by a huge atmosphere composed mostly of hydrogen. And perhaps it has a larger core, surrounded by a deep ocean of water above which is a thin atmosphere rich in water vapor. If the only data we have is density, it is impossible to distinguish between these two possibilities. There is no doubt that the possibility of an ocean is of course more exciting, given the fact that liquid water is considered a prerequisite, and perhaps even necessary, for the existence of life in the form we know.

And yet, when Jacob Bean, an astronomer at the University of Chicago, observed the planet at several different wavelengths, hoping to see a change in its apparent size that could indicate the thickness of the atmosphere, he saw nothing. This can be explained in two ways. The planet may have an inflated hydrogen atmosphere, but is full of clouds and haze that make it difficult to detect. Or it has a thin, watery atmosphere, but too thin for ground-based telescopes to accurately map its characteristics. This situation can be compared to looking at a mountain range in the distance, says Kreidberg, who started working with Bean last year. "Even if there are peaks, if you're too far away, they might look like a straight line."

To try to settle the issue, Dut and his colleagues were granted 60 laps of the Hubble telescope and they have already started making their observations. This isn't the first time astronomers have looked at GJ 1214b with Hubble, but this program is much more comprehensive than any of its predecessors and will take advantage of the powerful new camera, Wide Field Camera 3, installed on the telescope's last servicing mission in 2009. With any luck, this observational expedition will settle once and for all the question: is GJ 1214b a water world or not.

The hunt for oxygen

Today, when astronomers have been engaged in the matter of planet hunting for some time, they have started to find many more planets whose orbital period is long. These planets are at a greater distance from their star and are therefore colder than the earlier group of hot Jupiter-like planets. "For a long time, we were limited to planets whose temperature is from 1,500 degrees Kelvin to 2,000 degrees Kelvin, that is, really hot," says Knutson of Caltech. Under these conditions, "most of the carbon in the atmosphere binds to oxygen, creating carbon monoxide," she says. "The most interesting thing that happens when you go below 1,000 degrees Kelvin is that carbon combines instead within methane molecules (CH4).”

Methane is particularly noteworthy because it can be a sign of biological activity, albeit an ambiguous sign since methane can be produced by geophysical processes from start to finish. Oxygen, especially in the form of ozone (O3), a highly active molecule made of three oxygen atoms, would be a much more plausible signal for the presence of life. It is also very difficult to detect because its spectral signature is elusive, especially in the relatively small atmosphere of an Earth-sized planet.

And yet, all the frenzied activity around moderate-temperature super-Earths adds to the focus on the big prize. "This whole business is really just an exercise," says Seeger. "I mean, it's interesting in itself, but for people like me, it's just a stepping stone to the point where we finally move on from super-Earths and start exploring the atmospheres of Earth-sized planets."

That likely won't happen before the James Bev Space Telescope is launched into orbit, possibly in 2018, and before a new generation of huge ground-based instruments, such as the Large Magellan Telescope and the Thirty Meter Telescope, begin operating in 2020. But even with these powerful tools, Seeger says, "it would take hundreds upon hundreds of hours" of observation time. Even then it is not clear whether we will be able to identify the mark of life unequivocally; To that end, observers may still need the Terrestrial Planet Finder, the funding for which has been cut so painfully that, as of now, any hope of an actual launch date is little more than a wild guess.

However, it is worth noting that no one in the 90s of the 20th century dreamed at all that already at such an early stage Cigar could even talk about a real chance of finding biological signatures. We are no longer satisfied with the hope that an extraterrestrial culture will recognize us and send a message in our direction. We actually explore the air above distant worlds and search the skies above for signs that someone is home.


About the author

Michael D. Lemonick is a journalist for the non-profit news website, Climate Central, and the author of the book Mirror Land: The Search for Our Planet's Twin Brother (published by Volker Books, 2012). For 21 years he served as a science reporter for Time magazine.

in brief

Conventional wisdom held that it would be nearly impossible to study the atmospheres of distant planets in other solar systems because the light emanating from their parent star would be too bright.

However, once scientists began to study such planets as they pass behind their star, they realized that the change in the brightness of the star obtained in such a situation could provide clues to the materials of which the atmospheres are made.

Astronomers are now using such advanced methods to detect atoms and molecules in these atmospheres. They hope that they can soon expand their search to include molecules that will provide evidence of distant life.

Eclipse: The Spitzer Space Telescope is able to detect the tiny change in brightness that occurs when a planet passes behind its parent star. Credit: Caltech and NASA
Eclipse: The Spitzer Space Telescope is able to detect the tiny change in brightness that occurs when a planet passes behind its parent star. Credit: Caltech and NASA



how it works

Comparative photography

The ongoing search for exoplanets is trying to find them by detecting a characteristic dimming of the brightness of stars that occurs when their planets pass by them (right). But if you want to know what a planet's atmosphere is made of, you'll have to look for the second, weaker dimming that occurs when a planet passes behind its star. This eclipse blocks the reflected light from the star. By comparing the starlight before this back transit and during the transit it is possible to deduce the composition of the reflected light and build a picture of the molecular composition of the atmosphere (below).

find a planet

The Kepler space telescope has observed more than 100,000 nearby stars since 2009 in hopes of finding a dimming of a star's brightness that occurs when an occulting planet passes between the star and Earth. These eclipses will typically reduce the star's brightness by a factor of 1 in 10,000.

find an atmosphere

The distant planets are also supposed to return some of the starlight in our direction. The wavelength composition of the reflected light depends on the planet's atmosphere, because certain molecules in the atmosphere will absorb or reflect light at certain wavelengths. As a planet passes behind its star, astronomers measure the dimming in brightness that occurs as the reflected light disappears. If the astronomers follow this dimming at many different wavelengths, they can reconstruct from the resulting data the atmospheric composition of the planet.

As the article was being prepared for print, it became known that the Kepler space telescope broke down, this update is still true today

And more on the subject

Planets We Could Call Home. Dimitar D. Sasselov and Diana Valencia in Scientific American, Vol. 303, no. 2, pages 38-45; August 2010.


Exoplanet Atmospheres. Sara Seager and Drake Deming in Annual Review of Astronomy and Astrophysics, Vol. 48, pages 631-672; September 2010.

Kepler mission:


The article was published with the permission of Scientific American Israel

3 תגובות

  1. Now we have to wait for telescopes to photograph the planets themselves
    And also we will find a breakthrough for high speed
    The first saying could take place in 10 years
    The second statement cannot be known

  2. An example of building a model of a planet

    The discovery of planets outside the solar system and their great diversity from each other and from what we know, metaphorically create a zoo of planets. At the same time, the interest in extraterrestrial life is growing more and more. The very discovery of hundreds of planets increases the likelihood and even encourages more about finding life outside the Earth.

    The Cassini spacecraft orbiting Saturn has discovered that its moon Dione has its own atmosphere. Because of the very thinness of this atmosphere, the more appropriate term for it is ecosphere. What is special about the discovery is that the diameter of Dion is 1120 km. It is an atmosphere that cannot support clouds and certainly has no climatic phenomena.

    These two basic data can be used as a basis for building a model of a planet. According to this model, a planet with a diameter of 1000 km orbits the sun at a distance of 1.8 astronomical units and that its sun is the same as ours. Its density is 6.6, for comparison the Earth's density is 5.5. It rotates around itself once every 15 hours and its inclination angle is 12 degrees. This planet has one moon with a diameter of 300 km that orbits it once in 30 hours and at a distance of 30,000 km from it.
    The first immediate conclusion is that this moon shows this planet only one side of it. Two days of the planet are equal to one day of the moon. This planet has solar and lunar eclipses similar to those of Earth. Considering their relative size differences, it is clear that the Moon exerts tidal forces on the planet like our Moon does on Earth.

    Such a small planet with a large density could hold an atmosphere. Two additional data are entered into the model. The density of the atmosphere is 0.8 that of the Earth and the atmospheric pressure is 0.6 bar. It is likely that, as we said earlier, there will also be climatic changes and winds. For comparison, the density of the atmosphere of Mars is 1% of that of the Earth, the atmospheric pressure is 7 millibars and strong winds blow on it reaching a speed of 480 km/h. It is therefore probable that winds blow on this planet as well. It could be that the size of the planet has a lot of meaning in relation to the strength of the winds. As an example, we will insert another data into the model. Wind speed reaching up to 300 km/h. What is their effect on the development of hurricanes, etc.? How are Coriolis forces affected by this and with what intensity?

    Is there liquid water on it? Considering the density of the atmosphere it could be yes. We will now enter another figure for this planet, two oceans. The depth of one of them is 2 km and the other 3 km and several rivers are tens and hundreds of kilometers long and tens of meters wide. Hurricanes and typhoons can develop in the oceans. If the planet has active tectonics, what is the chance that tsunami waves will develop on it? Since this planet is more distant than Earth from the sun, the solar constant is smaller and the illuminance of the sun is smaller. Since it is tilted on its side, it has cyclic climatic phenomena. Since its angle of inclination is smaller, the latitudes are higher than those of the Earth more will be found throughout the day either in total darkness in winter or in full daylight in summer. The zenith area, what is known on Earth as the circle of Capricorn and the circle of Cancer, is 12°. North and South. On the longest day the sun will be at the zenith at these latitudes. On Earth, the sun's rays penetrate up to a depth of 200 meters in bodies of water. Since this planet is farther from the sun, the penetration depth of its rays into the bodies of water is smaller.

    This set of rocks is just an example of the conclusions that can be drawn from a number of basic data in any model of a planet. The range of possibilities is endless. The importance of this way of thinking is that it gives the researchers a methodological tool regarding the discoveries of additional planets and future possibilities of flying to them. Learning within certain limits what to expect.

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