Snowball Planets

By Judith E Braffman-Miller

Today, Earth is located comfortably within the inner edge of our Star's habitable zone, where conditions are favorable for the emergence and evolution of life as we know it. However, this has not always been the case. Indeed, some astronomers propose that in the early days of our Solar System, our planet was an enormous "snowball". According to the Snowball Earth model, our planet's surface was entirely--or almost entirely--frozen over at least once, sometime earlier than 650 million years ago. In May 2018, a team of planetary scientists announced their new findings that the ancient Earth was probably not the only jumbo "snowball" to inhabit the Cosmos. This is because aspects of an otherwise Earth-like planet's tilt and orbital dynamics can have a very destructive influence on its potential habitability, even triggering abrupt "snowball" eras where oceans freeze over and surface life is impossible.

According to new research from astronomers at the University of Washington, Seattle, merely locating a planet in its parent-star's habitable zone is not sufficient to designate it as a potentially life-friendly world. The habitable zone surrounding a star, like our own Sun, is that "Goldilocks" region of space where temperatures are not too hot, not too cold, but just right to encourage the emergence of life.

In our own Solar System, the planet Venus is a case in point. Venus has traditionally been designated as Earth's "twin", but if Venus is our planet's twin, it is an evil one. Even though Venus is located within that swath of interplanetary space considered to be the habitable zone surrounding our Sun, it is an Earth-sized ball of hell. Venus, even though it is almost the same size as our own comfortable planet, as well as its near neighbor in interplanetary space, is the victim of a runaway greenhouse effect. Venus is much hotter than it should be--in fact, it has the hottest surface of any planet in our Sun's family. It is even hotter than the innermost major planet, Mercury, that is situated much closer to our Star. This horrific heat causes the rocks on the surface of Venus to glow with an eerie red hue, similar to that of toaster coils. In addition, Venus's heavy blanketing atmosphere causes the pressure on its surface to be so extreme and merciless that any unfortunate life-form there would be instantly crushed.

Of course, extreme cold can be just as destructive as extreme heat. Earth experienced several Proterozoic Snowball (glaciation) periods in its early history.

Fire And Ice

Our Solar System emerged from assorted fragments left over from the long-dead nuclear-fusing cores of previous generations of ancient stars. Our Star, the Sun was born within a particularly dense blob embedded within a giant, cold, and dark molecular cloud. These beautiful, ghostly clouds float around our Milky Way Galaxy in great numbers, and they serve as the bizarre cradles of newborn stars (protostars). Although it may seem counterintuitive, things have to get very cold in order for a protostar to be born. In the secretive depths of these swirling clouds, composed of gas and dust, slender and fragile threads of material merge and clump together slowly, growing for hundreds of thousands of years. Ultimately, crushed together relentlessly by the intense squeeze of gravity, hydrogen atoms existing within this dense blob rapidly, dramatically, and suddenly fuse. This lights the protostar's stellar flames, that will rage brilliantly, casting starlight into the Universe for as long as the new baby star lives.

Our Galaxy's stars, our own Sun included, were all born this way--as the end result of the gravitational collapse of a particularly dense blob embedded in the undulating folds of a giant, frigid, dark molecular cloud.

Today, our Sun is a middle-aged, main-sequence (hydrogen-burning) star on the Hertzsprung-Russell Diagram of Stellar Evolution. As stars go, it is rather ordinary. It was born about 4.56 billion years ago, as a glittering newborn member of a dense open star cluster--along with thousands of other sparkling sibling stars. Many astronomers propose that our Sun was either unceremoniously evicted from its natal cluster, or that it simply peacefully drifted away from its stellar siblings about 4.5 billion years ago--when it was a mere youngster. The long-lost solar siblings have by now drifted off to more remote areas of our Milky Way. Like other open star clusters, our Sun's natal cluster fell apart as time passed.

Stars, like people, do not live forever. In another 5 billion years, or so, our Sun's starlight will go out. A star of our Sun's mass "lives" for approximately 10 billion years. As our Sun perishes, it will first evolve into a gigantic red giant star, before it finally winds up as a form of very dense stellar corpse called a white dwarf. A white dwarf is the relic core of a once-"living" progenitor star, and these very dense objects are surrounded by a beautiful, multicolored, gleaming shroud composed of gases that were once part of the progenitor star's outer layers of gas. Indeed, these objects--called planetary nebulae--are so beautiful that they are frequently referred to as the "butterflies" of the Universe.

As our Sun evolves into a bloated red giant, it will incinerate some of its planetary offspring--first Mercury, then Venus, and then possibly Earth. As our angry red, dying Sun swells to monstrous proportions, the habitable zone surrounding it will move outward. In the end, our bloated Sun will have swollen to the point that it will convert the frozen inhabitants of the Kuiper Belt into tropical paradises. The Kuiper Belt is currently the home of a multitude of frozen comet nuclei and icy dwarf planets--such as Pluto and its large moon Charon--and it circles our Sun beyond the orbit of the outermost major planet, Neptune.

Those astronomers who propose that our Earth was once a giant "snowball" argue that this model best explains the existence of certain sedimentary deposits, that are generally thought to be of glacial origin, at what were palaeolatitudes--as well as some other unexplained features that exist in Earth's geological record. However, scientists who do not favor the Snowball Earth hypothesis think that the implications of the geological evidence for global glaciation, and the geophysical likelihood of an ice or slush-coated ancient ocean, are not very convincing. The opponents of the Snowball Earth model emphasize the difficulty a planet would confront as it attempts to escape from this all-frozen state. A number of mysteries remain to be solved, including whether the Earth was entirely a jumbo "snowball", or was instead a big ball of slush, sporting a slender equatorial band of open water.

The Snowball-Earth eras are thought to have occurred before the rapid spread of multicellular life-forms on our planet, termed the Cambrian explosion. The most recent Snowball era may have actually triggered the evolution of multicellular bioforms--but another, much more ancient and longer Snowball era--called the Huronian glaciation (which would have occurred between 2300 to 2100 million years ago) may have resulted from the first appearance of oxygen in our ancient planet's atmosphere. This is referred to as the Great Oxygenation Event.

Snowballs In Space

Dr. Russell Deitrick, a post-doctoral researcher at the University of Bern in Switzerland, explained in a May 14, 2018 University of Washington Press Release that he and his team had set out to learn, using supercomputer modeling, how two features--a planet's obliquity or its orbital eccentricity--might play a role in its potential for the emergence of life. The astronomers limited their investigation to planets orbiting within the habitable zones of G dwarf stars--which are those like our Sun. Dr. Deitrick, who did his work with the University of Washington, is lead author of a paper describing this new study to be published in the Astronomical Journal. Dr. Deitrick's co-authors--all of the University of Washington--are atmospheric sciences professor Dr. Cecillia Bitz, astronomy professors Dr. Rory Barnes, Dr. Victoria Meadows and Dr. Thomas Quinn and doctoral student David Fleming, with additional help from undergraduate researcher Caitlyn Wilhelm.

A planet's obliquity refers to its tilt relative to the orbital axis, which is what determines the planet's seasons. Orbital eccentricity refers to the shape, and how circular or elliptical (out of round, or oval) the planet's orbit is. With elliptical orbits, the distance to the parent-star alters as the planet travels closer to, and then further from, its stellar parent.

Our own Earth is the only planet that is actually known, at least at present, to host life successfully, as it orbits our Star at an axial tilt of approximately 23.5 degrees--altering only very little over the millennia. However, Dr. Deitrick and his team asked the important question in their new model: What if those tiny alterations were greater for an Earthlike planet in orbit around a star similar to our own Sun?

Earlier research studies suggested that a greater axial tilt, or a tilting orbit, for a planet circling in a Sunlike star's habitable zone (that also orbits its parent-star at the same distance Earth does our Sun) would make the world warmer. For this reason, Dr. Deitrick and his colleagues were surprised when they discovered, through their supercomputer modeling, that the opposite actually seems to be the case.

"We found that planets in the habitable zone could abruptly enter 'snowball' states if the eccentricity or the semi-major axis variations--changes in the distance between a planet and star over an orbit--were large or if the planet's obliquity increased beyond 35 degrees," Dr. Deitrick explained in a May 14, 2018 University of Washington Press Release.

The new research is important because it aids astronomers in sorting out conflicting scenarios that were proposed in the past. It also makes use of a sophisticated method of ice sheet growth and retreat in the planetary modeling. This provides a significant improvement over some earlier studies, co-author Dr. Rory Barnes commented in the same University of Washington Press Release.

"While past investigations found that high obliquity and obliquity variations tended to warm planets, using this new approach, the team finds that large obliquity variations are more likely to freeze the planetary surface. Only a fraction of the time can the obliquity cycles increase habitable planet temperatures," Dr. Barnes continued to explain.

Dr. Barnes added that Dr. Deitrick "has essentially shown that ice ages on exoplanets can be much more severe than on Earth, that orbital dynamics can be a major driver of habitability and that the habitable zone is insufficient to characterize a planet's habitability." He continued to note that the new research also suggests "that the Earth may be a relatively calm planet, climate-wise."

This new form of modeling can also serve the valuable function of helping astronomers decide which planets, that circle stars beyond our Sun, are the best targets for observation--worthy of taking up precious telescope time. "If we have a planet that looks like it might be Earth-like, for example, but modeling shows that its orbit and obliquity oscillate like crazy, another planet might be better for follow-up" with future telescopes, Dr. Deitrick explained in the May 14, 2018 University of Washington Press Release.

Dr. Deitrick added that the primary value of the research is that "We shouldn't neglect orbital dynamics in habitability studies."

Other co-authors of the study are Dr. Benjamin Charnay (LESIA Observatoire de Paris) and Dr. John Armstrong (Weber State University).

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.