Elizabeth Tasker

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.

I started reading this as I grieved a friend I'd talked about astronomy with over the years.

My love of astronomy was cemented as a core piece of my personality the Summer of 1995 at the month long Governor's School @ William and Mary University. Many of the concepts touched on in this book were first encountered then.

For many years after I finished my physics BS and moved out to California my Mom bought me popular science books like this.

All of that was in the mix every time I read a show more chapter over the course of 2019. The running theme that blows my mind about the science of exo-planet hunting is how much information is extracted from such tiny data points. A dip in brightness of a star says so much. I feel the author did the same thing with the topic.

Most of us have a picture in our mind of a planet orbiting our sun. Some of us really think about the moon whenever we see it in the sky. This book provides the birds and bees of how planets are made. It uses that foundation to breakdown the exo-planet headlines over the last few decades. It reminds us how many projects and devices have contributed to this particular slice of astronomy.

It's not lost on me that many of the topics hint at how we might make our planet uninhabitable in the coming centur(ies). There are those who dismiss climate science and one of the many reasons I pity such individuals is that they'll never get the chance to appreciate this book.

I tend to not binge popular science books. I read a chapter and then put it aside for a week or a month. Many don't survive the process and are left unfinished. This one, I'm tempted to start a second read-through in the new year. show less

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 show 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. show less

The Planet Factory was one of our family story time reads, and let me tell you, it was interesting reading a lot of those exoplanet names out loud. Exoplanet names are typically catalogue style, with initials representing the method used to find the planet, a number representing the star it is orbiting, and then another letter to represent which object it is in its star system — they are alphanumeric soup, some of them quite long.

This is clearly not a book meant for reading aloud, but we show more are not going to hold that against it. It was interesting to learn about planetary formation and they ways that our solar system both may and may not be very representative of most systems in the universe.

Could be a very interesting reference for speculative fiction writers interested in planetary level worldbuilding!

Didn’t always hold my youngest child’s interest, but my oldest was into it, and and it often inspired physics discussions with my husband. show less