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The Planet Factory: Exoplanets and the Search for a Second Earth (2017)

por Elizabeth Tasker

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"The Planet Factory tells the story of exoplanets, planets orbiting stars outside of our solar system. Discover the specks of dust that circle a young star come together in a violent building project that can form colossal worlds hundreds of times the size of the Earth; the changing orbits of young planets that risk dooming the life forming on neighboring worlds or, alternatively, that can deliver the key ingredients needed to seed its beginnings. Exoplanets are one of the greatest construction schemes in the universe and they occur around nearly every star you see. Each result is an alien landscape, but is it possible that one of these could be like our own home? The Planet Factory discusses the way these planets form, their structure and features, and describes in detail the detection techniques used (there are many) before looking at what we can learn about the surface environments and planetary atmospheres, and whether this hints at the tantalizing possibility of life." --… (más)
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This is an excellent survey of exoplanet science, for a general audience.

My main complaint is that Tasker too often tells a story, e.g., of exoplanet formation, without telling us how we know that story or what the uncertainties are. The process of science is as important as the conclusions, and Tasker sometimes shortchanges us. Not always, though—I just wanted more details. Although the book is very well organized, I would have liked to learn more of Tasker's own (presumably more specialized) research. Her webpage says, "My research uses computer models to explore the formation of planets and galaxies," but this doesn't come out at all in the book. There is some description of the techniques for discovering exoplanets, but nothing on computer modeling.

More typos than I like ("numeracy," incorrect Celsius/Fahrenheit conversions, …).

> Should the estimated location of the Solar System's centre of mass be wrong, the true distance between the Earth and pulsar will be slightly off and the pulsar timings will appear irregular. In 2005, the arrival times of pulsar signals were scrutinised for evidence of an anomalous variation that would indicate a missing planet. They came up blank

> Modelling virtual systems that begin with five gas giants turns out to be more successful at reproducing our own Solar System than with just the big four. The extra planet forms just past Saturn with a similar size to the two icy worlds, Uranus and Neptune. During the ensuing chaos of planet rearrangement, this extra world passes slightly too close to Jupiter, which aggressively boots it from the Solar System.

> Although the carbon-silicate thermostat allows the Earth to compensate for small changes in the Sun's radiation, there is a limit. If the Earth is pushed too close to the Sun, the increase in water vapor cannot be compensated for swiftly enough by the reduction in carbon dioxide. The planet therefore heats still further and more water is evaporated into the atmosphere to boost the greenhouse effect. As temperatures pass 100°C (212°F), it stops raining and carbon dioxide removal is throttled. The greenhouse gases continue to climb through water evaporation and volcanic activity, raising the temperature continuously higher. Carbon is baked out of the rocks to escape into the air and react with oxygen to add still more carbon dioxide to the atmosphere. A runaway cycle ensues whereby the planet continues to get hotter and hotter until all the water is gone from the surface. This is probably the fate Venus met.

> This fails once the temperature drops sufficiently for the carbon dioxide to condense into clouds. Clouds of carbon dioxide reflect and block more of the Sun's diminishing heat to boost, rather than counter, the planet's cooling. The surface temperature on a more distant Earth would drop to zero at 1.4–1.7au. This is the point known as the Maximum Greenhouse limit

> In our Solar System, the temperate zone extends from 0.95au to 0.14au for a conservative estimate, and 0.84au to 1.7au at the generous edge.

> the temperate zone is planet dependent. Boost the Earth's carbon dioxide by a factor of 10, and even our current position might be unable to support liquid water. A different mix of gases in the atmosphere or different rocks would lead to a completely different cycle from the one on our home world.

> As the star fuses hydrogen into helium and on into the heavier elements, its core contracts. The contraction releases energy and the star brightens. Around 3 billion to 4 billion years ago, our Sun was 30 per cent fainter than it is today. Such a decrease in solar energy should have meant that our planet was 20°C (68°F) cooler than at the present time, and it was largely frozen. Rather confusingly, geological evidence suggests that the Earth had plenty of liquid water on its surface 4 billion years ago. Sedimentary rocks from this epoch have been found that could only have been created by solid particles settling out of a liquid. This problem is known as the faint young Sun paradox. The solution to this issue is still debated. One possibility is that our atmosphere was very different billions of years ago, containing a higher fraction of greenhouse gases capable of holding heat. The carbon-silicate cycle may have allowed carbon dioxide levels to rise as high as 80 per cent of the atmosphere mass. Alternatively, early bacterial life forms may have produced a high content of methane.

> As Mars's core cooled, the convection flow between the mantle and core stopped. Any plate tectonics ground to a halt and the magnetic field died. The magnetic field may have been given an extra death push by a major collision with a moon-sized object that struck Mars more than 4 billion years ago. At only few hundred million years old, the young planet was smacked so hard that it created a dichotomy between the two hemispheres of the Martian crust. The northern surface of the planet is an average of 5.5km (3.4mi) lower than the southern surface, with a crust that is 26km (16mi) thinner. A collision of this magnitude would have generated a huge amount of heat on the impacted northern side, resulting in a temperature gradient across the planet. This might have disrupted the convection flow through the planet's mantle and throttled the magnetic field. The soaring temperatures at the impact site would have demagnetised the rocks in that area, explaining why the magnetised rocks are found predominantly on the planet's southern surface.

> Further observations of Gliese 581 had recorded unusual magnetic activity on the star's surface. The magnetised patch was similar to a sunspot and was interfering with the surrounding flow of stellar material. As the star rotated, the spot appeared as a periodic wobble that looked strongly like the influence of a planet. When this effect was removed from the data, Gliese 581g vanished. What was worse, the correction also erased Gliese 581d.

> There are two problems with finding a transiting Earth-sized planet in the temperate zone. The first is that a planet and star analogue to our own has only a 0.1 per cent chance of transiting. From most viewing angles, the small and distant Earth does not cross the Sun's face. The second issue is that the decrease in light as the planet crosses the star is just one part in 10,000

> Unlike larger Sun-sized stars, a forming red dwarf can be 100 times brighter than its normal dim value once it begins to burn hydrogen into helium. If a planet has formed during this early phase, any surface water could be evaporated away before the star cools.

> the speed of the planetesimals and embryos. These rocky bodies move rapidly on close-in orbits. The final planet-formation stage may end up being dominated by high-velocity collisions, capable of stripping away atmosphere and water from a young world.

> A pure iron Earth-sized world would have a mass of nearly 4 Earths, while one dominated by ice might only weigh in at 0.32 Earths

> Based on the 2,300 planets that had been discovered by the Kepler Space Telescope by 2013, it was estimated that one in six stars had a planet within 80–125 per cent of the size of the Earth. Around the Milky Way's 100 billion stars, this would mean that 17 billion Earth-sized worlds are out there.

> From an examination of nearly 4,000 dwarf stars, just under 40 per cent have a planet that is likely to be rocky; 15 per cent of these were also within the temperate zone of the star.

> A 10 Earth mass rocky planet risks not having exposed continents unless it is very dry, with at least 10 times less water than our own planet. A larger version of our own Earth may therefore always be a water world.

> The Earth's seas have absorbed 10 times more carbon dioxide than is present in the air. This would also happen on an ocean world, but it turns out that the mechanism is the Devil's thermostat. Sea absorption of carbon dioxide is most efficient when the temperature is cooler. If the planet's temperature rises, the sea will draw less carbon dioxide from the atmosphere and allow more heat to be trapped. Should the reverse happen and the planet cools, the sea will increase the removal of carbon dioxide and let more heat escape. Rather than countering a change in the planet's temperature, the endless oceans will accelerate it.

> we do not know how much water is stored in the Earth's mantle. If it is approximately the same quantity as that of the surface oceans, then a 10 Earth-mass planet with plate tectonics could avoid a water world's fate

> Oxygen flooded the atmosphere in a juncture referred to as the Great Oxygenation Event. Unfortunately, most of the populations on the early Earth were anaerobic bacteria that have a toxic response to oxygen. They died in droves, wiping out a huge chunk of life on the planet. Meanwhile, the oxygen in the atmosphere reacted with methane to produce more carbon dioxide and water vapor. While both these products are greenhouse gases, neither is as effective at trapping heat as methane. The removal of methane therefore caused the temperature on the Earth to plummet, producing the massive Huronian glaciation that is our oldest known ice age. During this time, our planet may have become almost entirely frozen, creating a snowball Earth. ( )
  breic | Sep 7, 2020 |
Striking a good balance between technical detail and narrative engagement this situation report on what we know about the processes that create planets simply reinforces the notion that to get a world like Earth, that can support complex life, would seem to be an uncommon event. The suggestion at the end is that we're something like being 25 years away from potentially identifying such a life-bearing planet does not fill me with hope, considering the state of contemporary international politics. ( )
  Shrike58 | Aug 25, 2019 |
Interesting book that summarizes the state of exoplanet science and discovery, discusses the questions that the discoveries triggered and offers some answers.
Well-written, in a language that keeps you reading, and that can also be understood by people not directly involved with astronomy. ( )
  bluyssae | Apr 19, 2019 |
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"The Planet Factory tells the story of exoplanets, planets orbiting stars outside of our solar system. Discover the specks of dust that circle a young star come together in a violent building project that can form colossal worlds hundreds of times the size of the Earth; the changing orbits of young planets that risk dooming the life forming on neighboring worlds or, alternatively, that can deliver the key ingredients needed to seed its beginnings. Exoplanets are one of the greatest construction schemes in the universe and they occur around nearly every star you see. Each result is an alien landscape, but is it possible that one of these could be like our own home? The Planet Factory discusses the way these planets form, their structure and features, and describes in detail the detection techniques used (there are many) before looking at what we can learn about the surface environments and planetary atmospheres, and whether this hints at the tantalizing possibility of life." --

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