Planet Earth and exoplanets in the so-called habitable zone
It has long been thought that our planet resides in a more or less perfect situation to support life in its orbit around the sun, an orbit oft been described as being in the goldilocks zone – not too hot, not too cold; indeed just perfect for supporting life on this planet, and certainly, as René Heller, a postdoctoral fellow at the Origins Institute at McMaster University in Ontario, has suggested[1], our planet does possess a number of properties that at first glance seem ideal for life.
Firstly, says Heller, it revolves around a sedate, middle-aged star that has shone steadily for billions of years, giving life plenty of time to arise and evolve. Secondly, it has oceans of life-giving water, largely because it orbits within the sun’s “habitable zone”, a slender region where our star’s light is neither too intense nor too weak. Inward of the zone, a planet’s water would boil into steam; outward of the area, it would freeze into ice. Thirdly, Earth has a life-friendly size; big enough to hold onto a substantial atmosphere with its gravitational field, but small enough to ensure that gravity does not pull a smothering, opaque shroud of gas over the planet. And fourthly, Earth’s size and its rocky composition also give rise to other boosters of habitability, such as climate regulating plate-tectonics and a magnetic field that protect the biosphere from harmful cosmic radiation. The moon is likely behind both these factors and it also prevents the earth from tipping too far on its axis.[1.3]
Also as John Gribbin points out[1.5], our sun’s Goldilocks situation in the Milky Way also derives from the fact that it is not too close to the galactic centre, where stars are more crowded and dangerous events such as supernovae and gamma-ray bursts are common, and not too far, where stars are too sparse for enough metals (in astronomical terms, the heavy metals include everything heavier than hydrogen and helium) to build up to form rocky planets. According to Charles H. Lineweaver of the Australian National University, there is a “galactic habitable zone” extending from about 23,000 to 30,000 light-years from the galactic centre, and the sun is situated close to the middle. This habitable zone comprises only about 7 percent of the galactic radius, containing fewer than 5 percent of the stars because of the way they are concentrated toward the core. The orderly arrangement of planets in nearly circular orbits in our solar system thereby providing long-term stability is uncommon. Most planetary systems are chaotic places, lacking the calm Earth has provided for life to evolve.
Yet, as Heller elaborates, the more closely scientists study Earth’s habitability, the less ideal it appears to be. Habitability varies widely across Earth, with the result that large portions of its surface are relatively devoid of life, consisting of arid deserts, the nutrient-poor open ocean and the frozen polar regions. Earth’s habitability also varies over time. During the Carboniferous period from about 350 to 300 million years ago, the planet’s atmosphere was warmer, wetter and far more oxygen rich than it is now. Crustaceans, fish and reef-building corals flourished in the seas, great forests blanketed the continents, and insects and other terrestrial creatures grew to gigantic sizes. Because the Carboniferous Earth appears to have supported significantly more biomass than our present-day planet, Earth today is less habitable than it appears to have been at times in its ancient past.
And as Heller goes on to project, Earth will certainly become far less life-friendly in the future. About 5 billion years from now, our sun will have largely exhausted its hydrogen fuel and begun fusing more energetic helium in its core, causing it to swell to become a “red giant” star that will scorch Earth to a cinder. However, long before that, life on Earth should already have come to an end. As the sun burns through its hydrogen, the temperature at its core will gradually rise, causing our star’s total luminosity to slowly increase, brightening by about 10% every billion years. Such change means that the sun’s habitable zone is not static but dynamic, so that over time, as it sweeps father out from our brightening star, it will eventually leave earth behind. To make matter worse, recent calculations suggest that Earth is not in the middle of the habitable zone but rather on the zone’s inner cusp, already teetering on the edge of overheating, as depicted at right.
“Consequently, within about half a billion years, our sun will be bright enough to give earth a feverish climate that will threaten the survival of complex multicellular life. By some 1.75 billion years from now, the steadily brightening star will make our world hot enough for the oceans to evaporate, exterminating any simple life lingering on the surface. In fact, Earth is well past its habitable prime, and the biosphere is fast approaching its denouement. All things considered, it seems reasonable to say that our planet is at present only marginally habitable”[2].
Since the first exoplanetary transit was detected in 1999, as at 18 December 2017, some 3,567 have been confirmed, mostly by transit across their stars, mostly with the aid of the Kepler space probe [3] or by other means, such as the Doppler effect. Statistics suggest that our galaxy harbours at least 100 billion more. [3.1]
As a matter of definition, the Earth-like planets being searched for encapsulate planets of similar size and mass to the Earth that could plausibly have oceans of liquid water. Such a planet must be located within the area around its star where the star's heat would be strong enough to melt water ice but not vapourise it. In other words, the planet's habitable zone capable of supporting life.
Kepler is about to shut down, to be replaced by several new missions targeting exoplanets to launch in 2018: The Transiting Exoplanet Survey Satellite (TESS) and the Cracterising Exoplanet Satellite (CHEOPS). As with Kepler, these will both search for signs of planets crossing in front of their parent stars, producing a characteristic dimming of the parent stars' brightness, ever so small and ever so brief. Per medium of this dimming, astronomers can identify planets that are too faint to see on their own.
Another technique is the Doppler effect, already noted in passing. If Doppler shifting shows that a star moves back and forth in a regular pattern, a planet's gravitational pull must be tugging the star in and out as it orbits around. A blue-shifted short wavelength indicates an approaching star, and a red-shifted long wavelength a retreating star. Once TESS identifies a candidate planet, CHEOPS will zero in for a closer examination.
Generally speaking, TESS is searching for planets that orbit very quickly, in a few weeks or less, and most of these will be red dwarfs whose habitable zone lies very close to the star where orbital periods are short. In fact. most of the stars in our galaxy are so-called red dwarfs, cooler and fainter stars with less than half the mass of the sun.
Another factor astronomers take into consideration on the habitable front is the orbiting planets' atmospheres as gleaned through the technique of spectroscopy. When a planet eclipses its star, some starlight passes through the planet's atmosphere on its way to Earth. Each atom or molecule has certain favourite wavelengths of light that it absorbs or deflects in other directions, depending on the energies of their electrons. The technique is to monitor the spectrum of the star, before, during and after a transit. By looking at the refracted light signals through coloured filters and comparing which wavelengths come through when a planet is blocking the star and when it is off to the side, scientists can get some idea of the composition of the planet's atmosphere.
Returning to Heller's earlier paper on the habitable zone, Heller’s choice for a so-called superhabitable world is a planet slightly larger than Earth orbiting a star somewhat smaller and dimmer than the sun known as a K dwarf star. [3.2] K dwarfs have less total nuclear fuel to burn than more massive stars, but they use their fuel more efficiently, thereby increasing their longevity. The middle age K dwarfs we observe today are billions of years older than the sun and will still be shining billions of years after our star has expired, Any potential biospheres on their planets would have much more time in which to evolve and diversify.
A K dwarf's light would appear somewhat ruddier than the sun’s, as it would be shifted more towards the infrared, but is spectral range would nonetheless support photosynthesis on a planet’s surface. Being longer-lived than our sun yet not treacherously dim, K dwarfs appear to reside in the sweet spot of stellar superhabitability. To be superhabitable, exoplanets around small, long-lived stars would need to be more massive than Earth, ideally about twice the size, to avoid its core going cold and causing the collapse of any protective magnetic field.
A magnetic field, generated by a spinning, convecting core of molten iron, acts like a dynamo. The core remains liquefied because of heat left over from the planet’s formation, as well as from the decay of radioactive isotopes. If this internal heat reservoir is lost, the core solidifies, the dynamo ceases to work, allowing cosmic radiation and stellar flares to erode the upper atmosphere and impinge on the surface, causing higher levels of damaging radiation harmful to surface life. A planet’s internal heat also drives volcanic eruptions and plate tectonics, processes that replenish and recycle atmospheric levels of the green house carbon dioxide, without which Earth like planets would enter an uninhabitable “snowball” state in which all of its surface water freezes.
A rocky-superhabitable super-Earth would have higher surface gravity, which could lead to a thicker atmosphere, more erosive weather and flatter topography. The result could be an “archipelago world” of shallow seas dotted with island chains rather than a more familiar world of deep oceans and large continents. Such geography could benefit life, given that Earth’s scattered archipelagos are among the most biologically dense and diverse spots on the planet.
Taken together these factors suggest that the notion of superhabitability presupposes a world slightly larger than earth with host stars somewhat smaller and dimmer than the sun. But notwithstanding its known imperfections our familiar Earth has served us well up until now and is certainly the best and only thing we have going for us at the moment[4].
Before leaving this topic, we must not lose the opportunity to refer to another kind of star, known as an ultracool dwarf. With the aid of the 60 centimetre TRAPPIST telescope situated in the Chilean desert has recently been discovered just 40 light years away. Small and red, it is known as TRAPPIST-1 and is about a 10th the size of our sun and a bit bigger than Jupiter, and has 3 planets orbiting it. Two are extremely close to the star, taking just 1.5 and 2.4 Earth days respectively to complete an orbit of their host.
Most exoplanet searches like those involving NASA’s Kepler telescope, concentrate on hotter, bigger stars, but TRAPPIST-1’s dimness makes it easier to look for life on these Earth-sized planets that would not be possible if they were orbiting brighter stars. To determine if a planet might harbour life, scientists usually analyse the makeup of its atmosphere. Biomarkers such as methane, oxygen and water could indicate life. For Earth-sized planets orbiting larger, brighter stars, the subtle signs of these molecules would be lost in the glare of the star’s light. However, in the faint glow of the red ultracool dwarf stars, the effect of the biomarkers should be big enough to be detected. So this kind of star represents the best target yet in the search for life beyond our solar system[5].
One important distinction
As John Gribbin points out[5.3], all this talk of Earth-like planets obscures another critical distinction. When astronomers say “Earth-like,” all they generally mean is a rocky planet in the habitable zone about the same size as ours. Venus would fulfil this definition, but you could never live there, and the fact that you can live on Earth is the result of fortuitous circumstances. The two planets differ in several important ways:
- Venus has a thick crust, no sign of plate tectonics and essentially no magnetic field. Earth has a thin, mobile crust where tectonic activity, especially around plate boundaries, brings material to the surface through volcanism.
- Over Earth's long history, this activity has carried ores up to where humans can mine them to provide the raw materials for our technological civilization.
- Plate tectonics has also brought nutrients to the surface to replenish those that get depleted by the cells living there, and it is crucial for recycling carbon and stabilizing the temperature over long timescales.
- Earth also has a large metallic core that, coupled with its rapid rotation, produces a strong magnetic field to shield its surface from harmful cosmic radiation. Without this screen, our atmosphere would probably erode, and any living thing on the surface would get fried.
"All these attributes of our planet are directly related to our moon—another feature that Venus and many other Earth-like planets lack. Scientists' best guess is that the moon formed early in the solar system's history, when a Mars-size object struck the nascent Earth a glancing blow that caused both protoplanets to melt.
"The metallic material from the two objects settled into Earth's center, and much of our planet's original lighter rocky material splashed out to become the moon, leaving Earth with a thinner crust than before. Without that impact, Earth would be a sterile lump of rock like Venus, lacking a magnetic field and plate tectonics. The presence of such a large moon has also acted as a stabilizer for our planet. Over the millennia Earth has wobbled on its axis as it goes around the sun, but thanks to the gravitational influence of the moon, it can never topple far from the vertical, as seems to have happened with Mars. It is impossible to say how often such impacts occur to form double systems such as Earth and its moon. But clearly they are rare, and without our satellite we would likely not be here"[5.5].
So, what does the future hold?
Any forecast must be advanced in probabilities. Scientists are as certain as they can be that the four giant planets have finished wandering and will still be on the same orbits five billion years from now, when the aging sun is expected to balloon outward and engulf the inner planets, but it’s a little bit less certain that the inner planets—Mercury, Venus, Earth, and Mars—will still be around to die that way.
There is a one percent chance the inner solar system will go dramatically unstable during the next five billion years, the problem being a weird long-distance connection between Jupiter and Mercury. When Jupiter’s closest approach to the sun lines up with Mercury’s noticeably squashed orbit in just the right way, Jupiter exerts a slight but steady tug. Over billions of years this gives Mercury a 1-in-100 chance of crossing the orbit of Venus. There is a further 1-in-500 chance that if Mercury goes unstable, it will also perturb the orbit of Venus or Mars enough for one of them to hit Earth—or miss it by several thousand miles, which would be almost as bad, since the entire Earth would get stretched and "melted like taffy"[6].
The faint risk of apocalypse - a 1-in-50,000 chance that the Earth will succumb to orbital chaos before the sun incinerates it - is our legacy of the solar system’s youth, when it turned inside out.
[1] In an article entitled “Better than Earth”, Scientific American, January 2015, 20 at 22. The graphic is on page 26.
[1.3] John Gribbin, "Alone in the Milky Way - Why we are probably the only intelligent life in the galaxy", Scientific American, September 2015, 86-91, at 88.
[1.5] Ibid.
[2] René Heller, "Better than Earth", op cit, (my emphasis).
[3] Launched in May 2009, the world’s first mission to search for exoplanets, and now (2018) coming to the end of its working life, to be replaced by TESS and CHEOPS mentioned above.
[3.1] This material on TESS, CHEOPS and the spectrometry techniques of atmospheric detection is drawn from Joshua N Winn's article "Shadows of other worlds", Scientific American, March 2018, 26 ff.
[3.2] For the somewhat fuzzy distinction between red dwarfs and K-dwarf stars, see en.wikipedia.org/wiki/Red_dwarf
[4] From René Heller, op cit, 23-27.
[5] For the foregoing, see Deborah Netburn,“Earth-like planets found in nearby system”, SMH 4 May 2016; also http://www.nasa.gov/feature/promising-worlds-found-around-nearby-ultra-cool-dwarf-star
[5.3] Gribbin, op cit at 90.
[5.5] Ibid.
[6] Source: David Jewitt and Edward D Young, “Oceans from the skies”, Scientific American, March 2015, 30 at 37. See also https://www.nationalgeographic.com/magazine/2013/07/125-solar-system/ These comments by Greg Laughlin of the University of California, Santa Cruz.