In 2016, the La Silla Observatory in Chile spotted evidence of possibly the most eagerly anticipated exoplanet in the Galaxy. It was a world orbiting the nearest star to the sun, Proxima Centauri, making this our closest possible exoplanet neighbour. Moreover, the planet might even be rocky and temperate.
Proxima Centauri b had been discovered by discerning a periodic wobble in the motion of the star. This revealed a planet with a minimum mass 30% larger than the Earth and an orbital period of 11.2 days. Around our sun, this would be a baking hot world.
But Proxima Centauri is a dim red dwarf star and bathes its closely orbiting planet in a level of radiation similar to that received by the Earth. If the true mass of the planet was close to the measured minimum mass, this meant Proxima Centauri b would likely be a rocky world orbiting within the habitable zone.
Sitting 4.2 light years from our sun, a journey to Proxima Centauri b is still prohibitively long.
But as our nearest neighbor, the exoplanet is a prime target for the upcoming generation of telescopes that will attempt to directly image small worlds. Its existence was also inspiration for privately funded projects to develop faster space travel for interstellar distances.
Yet observations taken around the same time as the La Silla Observatory discovery were painting a very different picture of Proxima Centauri. It was a star with issues.
This set of observations were taken with Evryscope; an array of small telescopes that was watching stars in the southern hemisphere. What Evryscope spotted was a flare from Proxima Centauri that was so bright that the dim red dwarf star became briefly visible to the naked eye.
Flares are the sudden brightening in the atmosphere of a star that release a strong burst of energy. They are often accompanied by a large expulsion of plasma from the star known as a “coronal mass ejection”. Flares from the sun are typically between 1027 – 1032 erg of energy, released in a few tens of minutes.
For comparison, a hydrogen bomb releases the equivalent of about 10 megatons of TNT or a mere 4 x 1023 erg. Hitting the Earth, energy from solar flares and coronal mass ejections can disrupt communication equipment and create a spectacular aurora.
But the Proxima super-flare spotted by Evryscope was well beyond a regular stellar flare.
On March 18 in 2016, this tiny red dwarf emitted an energy belch of 1033.5 erg. The flare consisted of one major event and three weaker ones and lasted approximately one hour, during which time Proxima Centauri became 68 times brighter.
A sudden, colossal increase in the brightness of a star does not bode well for any closely orbiting planets.
However, such a major flare might well be rare. If the star was normally fairly quiet, perhaps a planet could recover from a single very disruptive flare in the same way the Earth has survived mass extinction events.
Led by graduate student Ward Howard at the University of North Carolina, Chapel Hill, the discovering team used Evryscope to monitor Proxima Centauri for flares for a total of 1344 hours between January 2016 and March 2018. What they found was a horrifying environment, as reported in The Astrophysical Journal Letters.
While an event on the scale of the Proxima super-flare was only seen once, 24 large eruptions were spotted from the red dwarf, with energies from 1030.5 to 1032.4 erg. Allowing for the fact the star had only been observed for a small part of the year, this pattern of energy outbursts meant that a massive super-flare (1033 erg) was likely to occur at least five times annually.
But how important is this for the planet?
The Earth is protected from flares from our sun by our atmosphere. The ozone layer absorbs harmful ultraviolet radiation with wavelengths between about 2400 – 2800 Angstroms (10-10 m), preventing it reaching the surface. So what if Proxima Centauri b had a similar protective layer of gases as the Earth?
To answer this question, Howard and his team ran simulations of an Earth-like atmosphere on Proxima Centauri b.
As is the case for the sun, the team assumed that large flares would be frequently accompanied by a coronal mass ejection. Radiation and stellar material then flooded over an Earth-like Proxima Centauri b at the observed rate. And the atmosphere crumbled.
High energy particles in the coronal mass ejections split the nitrogen molecules (N2) in the atmosphere, which reacted with the ozone (O3) to form nitrogen oxide (NO2). After just 5 years, 90% of the ozone in the atmosphere was lost and the amount was still decreasing.
Without ozone, the surface of Proxima Centauri b would be stripped of its protection from UV radiation. During the Proxima super-flare, the radiation dose without the protective ozone would be 65 times larger than that needed to kill 90% of one of the most UV-resilient organisms on Earth.
“Life would have to undergo extreme adaptation to UV or exist underground or underwater,” Howard notes. “Only the most resistant organisms could survive on the surface in this environment.”
The simulation does assume that Proxima Centauri b does not have a magnetic field. Such a shield could channel the particles from the coronal mass ejection to the poles, forming the aurora as on Earth and reducing the damage to the atmosphere.
However, orbiting so close to the star, Proxima Centauri b is likely to be in tidal lock as the moon is to the Earth. This is expected to weaken the magnetic field, as the slower rotation makes it harder to create a magnetic dynamo within the planet.
So if the protective shields are lowered on Proxima Centauri b, is our nearest planet a world populated by highly resistant UV organisms? Or have we seen evidence that rather than warming the planet to allow life to exist, this star has snuffed it out?
Astronomers have found that Barnard’s star — a very close, fast-moving, and long studied red dwarf — has a super-Earth sized planet orbiting just beyond its habitable zone.
The discovery relied on data collected over many years using the tried-and-true radial velocity method, which searches for wobbles in the movement of the host star.
But this detection was something big for radial velocity astronomers because Barnard-b was among the smallest planet ever found using the technique, and it was the furthest out from its host star as well — orbiting its star every 233 days.
For more than a century, astronomers have studied Barnard’s star as the most likely place to find an extrasolar planet.
Ultimately, said Ignasi Rablis of Spain’s Institute of Space Studies of Catalonia, lead author of the paper in journal Nature, the discovery was the result of 771 observations, an extremely high number.
And now, he said, “after a very careful analysis, we are over 99 percent confident the planet is there.”
The planet is at least 3.2 times the size of Earth and orbits near the snowline of the system, where water cannot be expected to ever be liquid. That means is it a frozen world (an estimated -150 degrees Celsius) and highly unlikely to support life.
But Rablis and others on the large team say it also an extremely good candidate for future direct imaging and next-generation observing.
Thousands of exoplanets have been identified by now, and hundreds using the radial velocity method. But this one is different.
“Barnard’s star is the ‘great white whale’ of planet hunting,” said Paul Butler, senior scientist at the Carnegie Institution, a radial velocity pioneer, and one of the numerous authors of the paper.
Because the star is so close (but 6 light-years away) and as a result so tempting, it has been the subject of exoplanet searches for 100 years, Butler said. But until the radial velocity breakthroughs of the mid 1990s, the techniques used could not find a planet.
Nonetheless, an early exoplanet hunter, the Dutch-American astronomer Peter van de Kamp of Swarthmore College, thought that he had indeed found two gas giant planets around Barnard’s star in the 1960s. He used a different technique based on the movement of the host star, and the findings even made it into some textbooks. But later the detection was found to be incorrect.
Even after the modern exoplanet era began Barnard’s star kept its planetary secret close.
As Butler explained it, the combination of the planet’s size and distance from the star ultimately pushed the technology (and astronomers) to the very limit — requiring a measurement of 1.2 meters per second of “wobble.”
In contrast, the first planets were found by radial velocity that would detect 70 meter per second of wobble caused by the gravitational pull of a planet, and 30 years ago the best instruments could detect only 300 meters per second.
The detected planet (which remains a “candidate” until further confirmed) was ultimately found following concerted effort by a large team of astronomers around the world. It was co-led and organized by Guillem Anglada-Escudé of the Queen Mary University of London. The young astronomer had made a major splash in 2016 with the detection of a planet orbiting Proxima Centauri, the closest star to our own.
That discovery was part of the “Pale Red Dot” campaign, which had the goal of detecting rocky planets around red dwarf stars. After the Proxima discovery Barnard’s star went to the top of Anglada-Escudé list with the renamed “Red Dots” collaboration — which is supported by the European Southern Observatory and universities in Chile, the United Kingdom, Spain and Germany.
By 2015, there was already almost 18 years of modern data collected regarding a possible planet orbiting the star, and a faint but clearly present signal had been detected. But more was needed to confidently report a discovery, and the Red Dots effort took up the challenge.
To see if the result could be confirmed, astronomers regularly monitored Barnard’s star with high precision spectrometers such as the CARMENES (Calar Alto Observatory in Spain), and also the HARPS (High Accuracy Radial velocity Planet Searcher.)
Ultimately, the team used observations from seven different instruments taken over 20 years, making this one of the largest and most extensive datasets ever used for precise radial velocity studies.
“We all have worked very hard on this result,” said Anglada-Escudé. “This is the result of a large collaboration organized in the context of the Red Dots project, which is why it has contributions from teams all over the world including semi-professional astronomers.”
Cristina Rodríguez-López, researcher at the Instituto de Astrofísica de Andalucía and co-author of the paper, said of the significance of the finding grow over decades.
“This discovery means a boost to continue on searching for exoplanets around our closest stellar neighbors, in the hope that eventually we will come upon one that has the right conditions to host life,” she said.
The next pr0ject for the Red Dots campaign is to study the star Ross 154, at 9.69 light-years away another of the closest stars to us.
The dramatically increased (and increasing) precision in radial velocity measurements is expected to continue with the next generation of ground-based telescopes and spectrometers.
Butler, for instance, said that Carnegie is in the process of upgrading its Planet Finding Spectrograph at the Las Campanas Observatory in Chile to reach a 0.5-meters-per-second measurement. Other groups including the European Southern Observatory and American teams based at Pennsylvania State and Yale Universities have similar efforts under way.
If they succeed, Butler said, it may well be possible to find potentially habitable planets around sun-like and other categories of stars using the radial velocity method.
Barnard’s a very-low-mass red dwarf star in the constellation of Ophiuchus. It is the fourth-nearest-known individual star to the sun (after the three components of the Alpha Centauri system) and the closest star in the Northern Celestial hemisphere.
Despite its proximity, the star is too faint to be seen with the unaided eye, though it is quite visible with an amateur 8-inch telescope. It is much brighter in the infrared than in visible light. Although Barnard’s Star is an ancient star, it still experiences star flare events, one being observed in 1998.
The star is named after the American astronomer E. E. Barnard. He was not the first to observe the star (it appeared on Harvard University plates in 1888 and 1890), but in 1916 he measured its proper motion –the apparent angular motion of a star across the sky with respect to more distant stars — as 10.3 arcseconds per year relative to the sun.
This is likely to be the fastest star in terms of proper motion, as its proximity to the sun, as well as its high velocity, make it unlikely any faster object will be discovered.
Barnard’s Star is among the most studied red dwarfs because of its proximity and favorable location for observation near the celestial equator. Historically, research on Barnard’s Star has focused on measuring its stellar characteristics and its astrometry — which involves precise measurements of the positions and movements of stars and other celestial bodies on the plane of the sky.
When planet hunters use astrometry, they look for a minute but regular wobble in a star’s position as seen in images. Van de Kamp, for instance, used astrometry to study Barnard’s star and (incorrectly) detected those two gas giants around it.
In contrast, radial (or Doppler) velocities look for the wobble of the star perpendicular to the plane sky, and astronomers have regularly, and now once again, made history with that method.
What would happen if you switched the orbits of Mars and Venus? Would our solar system have more habitable worlds?
It was a question raised at the “Comparative Climatology of Terrestrial Planets III”; a meeting held in Houston at the end of August. It brought together scientists from disciplines that included astronomers, climate science, geophysics and biology to build a picture of what affects the environment on rocky worlds in our solar system and far beyond.
The question regarding Venus and Mars was proposed as a gedankenexperiment or “thought experiment”; a favorite of Albert Einstein to conceptually understand a topic. Dropping such a problem before the interdisciplinary group in Houston was meat before lions: the elements of this question were about to be ripped apart.
The Earth’s orbit is sandwiched between that of Venus and Mars, with Venus orbiting closer to the sun and Mars orbiting further out. While both our neighbors are rocky worlds, neither are top picks for holiday destinations.
Mars has a mass of just one-tenth that of Earth, with a thin atmosphere that is being stripped by the solar wind; a stream of high energy particles that flows from the sun. Without a significant blanket of gases to trap heat, temperatures on the Martian surface average at -80°F (-60°C). Notably, Mars orbits within the boundaries of the classical habitable zone (where an Earth-like planet could maintain surface water) but the tiny planet is not able to regulate its temperature as well as the Earth might in the same location.
Unlike Mars, Venus has nearly the same mass as the Earth. However, the planet is suffocated by a thick atmosphere consisting principally of carbon dioxide. The heat-trapping abilities of these gases soar surface temperatures to above a lead-melting 860°F (460°C).
But what if we could switch the orbits of these planets to put Mars on a warmer path and Venus on a cooler one? Would we find that we were no longer the only habitable world in the solar system?
“Modern Mars at Venus’s orbit would be fairly toasty by Earth standards,” suggests Chris Colose, a climate scientist based at the NASA Goddard Institute for Space Studies and who proposed the topic for discussion.
Dragging the current Mars into Venus’s orbit would increase the amount of sunlight hitting the red planet. As the thin atmosphere does little to affect the surface temperature, average conditions should rise to about 90°F (32°C), similar to the Earth’s tropics. However, Mars’s thin atmosphere continues to present a problem.
Colose noted that without a thicker atmosphere or ocean, heat would not be transported efficiently around Mars. This would lead to extreme seasons and temperature gradients between the day and night. Mars’s thin atmosphere produces a surface pressure of just 6 millibars, compared to 1 bar on Earth. At such low pressures, the boiling point of water plummets to leave all pure surface water frozen or vaporized.
Mars does have have ice caps consisting of frozen carbon dioxide, with more of the greenhouse gas sunk into the soils. A brief glimmer of hope for the small world arose in the discussion with the suggestion these would be released at the higher temperatures in Venus’s orbit, providing Mars with a thicker atmosphere.
However, recent research suggests there is not enough trapped carbon dioxide to provide a substantial atmosphere on Mars. In an article published in Nature Astronomy, Bruce Jakosky from the University of Colorado and Christopher Edwards at Northern Arizona University estimate that melting the ice caps would offer a maximum of a 15 millibars atmosphere.
The carbon dioxide trapped in the Martian rocks would require temperatures exceeding 300°C to be liberated, a value too high for Mars even at Venus’s orbit. 15 millibars doubles the pressure of the current atmosphere on Mars and surpasses the so-called “triple point” of water that should permit liquid water to exist. However, Jakosky and Edwards note that evaporation would be rapid in the dry martian air. Then we hit another problem: Mars is not good at holding onto atmosphere.
“Atmospheric loss would be faster at Venus’s current position as the solar wind dynamic pressure would increase,” said Chuanfei Dong from Princeton University, who had modeled atmospheric loss on Mars and extrasolar planets.
This “dynamic pressure” is the combination of the density of particles from the solar wind and their velocity. The velocity does not change greatly between Mars and Venus —explained Dong— but Venus’s closer proximity to the sun boosts the density by almost a factor of 4.5. This would mean that atmosphere on Mars would be lost even more rapidly than at its current position.
“I suspect it would just be a warmer rock,” Colose concluded.
While Mars seems to fare no better at Venus’s location, what if Venus were to be towed outwards to Mars’s current orbit? Situated in the habitable zone, would this Earth-sized planet cool-off to become a second habitable world?
Surprisingly, cooling Venus might not be as simple as reducing the sunlight. Venus has a very high albedo, meaning that the planet reflects roughly 75% of the radiation it receives. The stifling temperatures at the planet surface are due not to a high level of sunlight but to the thickness of the atmosphere. Conditions on the planet may therefore not be immediately affected if Venus orbited in Mars’s cooler location.
“Venus’s atmosphere is in equilibrium,” pointed out Kevin McGouldrick from the University of Colorado and contributing scientist to Japan’s Akatsuki mission to explore Venus’s atmosphere. “Meaning that its current structure does depend on the radiation from the sun. If you change that radiation then the atmosphere will eventually adjust but it’s not likely to be quick.”
Exactly what would happen to Venus’s 90 bar atmosphere in the long term is not obvious. It may be that the planet would slowly cool to more temperate conditions. Alternatively, the planet’s shiny albedo may decrease as the upper atmosphere cools. This would allow Venus to absorb a larger fraction of the radiation that reached its new orbit and help maintain the stifling surface conditions. To really cool the planet down, Venus may have to be dragged out beyond the habitable zone.
“Past about 1.3 au, carbon dioxide will begin to condense into clouds and also onto the surface as ice,” said Ramses Ramirez from the Earth-Life Sciences Institute (ELSI) in Tokyo, who specializes in modelling the edges of the habitable zone. (An “au” is an astronomical unit, which is the distance from our sun to Earth.)
Once carbon dioxide condenses, it can no longer act as a greenhouse gas and trap heat. Instead, the ice and clouds typically reflect heat away from the surface. This defines the outer edge of the classical habitable zone when the carbon dioxide should have mainly condensed out of the atmosphere at about 1.7 au. The result should be a rapid cooling for Venus. However, this outer limit for the habitable zone was calculated for an Earth-like atmosphere.
“Venus has other things going on in its atmosphere compared to Earth, such as sulphuric acid clouds,” noted Ramirez. “and it is much drier, so this point (where carbon dioxide condenses) may be different for Venus.”
If Venus was continually dragged outwards, even the planet’s considerable heat supply would become exhausted.
“If you flung Venus out of the solar system as a rogue planet, it would eventually cool-off!” pointed out Max Parks, a research assistant at NASA Goddard.
It seems that simply switching the orbits of the current Venus and Mars would not produce a second habitable world. But what if the two planets formed in opposite locations? Mars is unlikely to have fared any better, but would Venus have avoided forming its lead-melting atmosphere and become a second Earth?
At first glance, this seems very probable. If the Earth was pushed inwards to Venus’s orbit, then water would start to rapidly evaporate. Like carbon dioxide, water vapour is a greenhouse gas and helps trap heat. The planet’s temperature would therefore keep increasing in a runaway cycle until all water had evaporated. This “runaway greenhouse effect” is a possible history for Venus, explaining its horrifying surface conditions. If the planet had instead formed within the habitable zone, this runaway process should be avoided as it had been for the Earth.
“When I suggested this topic, I wondered whether two inhabited planets would exist (the Earth and Venus) if Mars and Venus formed in opposite locations,” Colose said. “Being at Mars’s orbit would avoid the runaway greenhouse and a Venus-sized planet wouldn’t have its atmosphere stripped as easily as Mars.”
But discussion within the group revealed that it is very hard to offer any guarantees that a planet will end up habitable. One example of the resultant roulette game is the planet crust. The crust of Venus is a continuous lid and not series of fragmented plates as on Earth. Our plates allow a process known as plate tectonics, whereby nutrients are cycled through the Earth’s surface and mantle to help support life. Yet, it is not clear why the Earth formed this way but Venus did not.
One theory is that the warmer Venusian crust healed breaks rapidly, preventing the formation of separate plates. However, research done by Matt Weller at the University of Texas suggests that the formation of plate tectonics might be predominantly down to luck. Small, random fluctuations might send two otherwise identical planets down different evolutionary paths, with one developing plate tectonics and the other a stagnant lid. If true, even forming the Earth in exactly the same position could result in a tectonic-less planet.
Venus’s warmer orbit may have shortened the time period in which plate tectonics could develop, but moving the planet to Mars’s orbit offers no guarantees of a nutrient-moving crust.
Yet whether plate tectonics is definitely needed for habitability is also not known. It was pointed out during the discussion that both Mars and Venus show signs of past volcanic activity, which might be enough action to produce a habitable surface under the right conditions.
“We reached the opposite conclusion to Bruce’s paper,” Dong noted cheerfully. “That is might be possible to use technology to give Mars an atmosphere. But it is fun to hear different voices and this is the reason why science is so interesting!”
Far beneath the frigid surface of the South Pole of Mars is probably the last place where you might expect the first large body of Martian liquid water would be found. It’s -170 F on the surface, there are no known geothermal sources that could warm the subterranean ice to make a meltwater lake, and the liquid water is calculated to be more than a mile below the surface.
Yet signs of that liquid water are what a team of Italian scientists detected — a finding that they say strongly suggests that there are other underground lakes and streams below the surface of Mars. In a Science journal article released today, the scientists described the subterranean lake they found as being about 20 kilometers in diameter.
The detection adds significantly to the long-studied and long-debated question of how much surface water was once on Mars, a subject that has major implications for the question of whether life ever existed on the planet.
Finding the subterranean lake points to not only a wetter early Mars, said co-author Enrico Flamini of the Italian space agency, but also to a Mars that had a water cycle that collected and delivered the liquid water. That would mean the presence of clouds, rain, evaporation, rivers, lakes and water to seep through surface cracks and pool underground.
Scientists have found many fossil waterways on Mars, minerals that can only be formed in the presence of water, and what might be the site of an ancient ocean.
But in terms of liquid water now on the planet, the record is thin. Drops of water collected on the leg of NASA’s Phoenix Lander after it touched down in 2008, and what some have described as briny water appears to be flowing down some steep slopes in summertime. Called recurrent slope lineae or RSLs, they appear at numerous locations when the temperatures rise and disappear when they drop.
This lake is different, however, and its detection is a major step forward in understanding the history of Mars.
The discovery was made analyzing echoes captured by the the radar instruments on the European Space Agency’s Mars Express, a satellite orbiting the planet since 2002. The data for this discovery was collected from observation made between 2012 and 2015.
Antarctic researchers have long used radar on aircraft to search for lakes beneath the thick glaciers and ice layers, and have found several hundred. The largest is Lake Vostok, which is the sixth largest lake on Earth in terms of volume of water. And it is two miles below the coldest spot on Earth.
So looking for a liquid lake below the southern pole of Mars wasn’t so peculiar after all. In fact, lead author Roberto Orosei of the Institute of Radioastronomy of Bologna, Italy said that it was the ability to detect subsurface water beneath the ice of Antarctica and Greenland that helped inspire the team to look at Mars.
There are a number of ways to keep water liquid in the deep subsurface even when it is surrounded by ice. As described by the Italian team and an accompanying Science Perspective article by Anja Diez of the Norwegian Polar Institute, the enormous pressure of the ice lowers the freezing point of water substantially.
Added to that pressure on Mars is the known presence of many salts, that the authors propose mix with the water to form a brine that lowers the freezing point further.
So the conditions are present for additional lakes and streams on Mars. And according to Flamini, solar system exploration manager for the Italian space agency, the team is confident there are more and some of them larger than the one detected. Finding them, however, is a difficult process and may be beyond the capabilities of the radar equipment now orbiting Mars.
The view that subsurface water is present on Mars is hardly new. Stephen Clifford, for many years a staff scientist at the Lunar and Planetary Institute, even wrote in 1987 that there could be liquid water at the base of the Martian poles due to the kind of high pressure environments he had studied in Greenland and Antarctica.
So you can imagine how gratifying it might be to learn, as he put it “of some evidence that shows that early theoretical work has some actual connection to reality.”
He considers the new findings to be “persuasive, but not definitive” — needing confirmation with other instruments.
Clifford’s wait has been long, indeed. Many observations by teams using myriad instruments over the years did not produce the results of the Italian team.
Their discovery of liquid water is based on receiving particularly strong radar echoes from the base of the southern polar ice — echoes consistent with the higher radar reflectivity of water (as opposed to ice or rock.)
After analyzing the data in some novels ways and going through the many possible explanations other than the presence of a lake, Orosei said that none fit the results they had. The explanation, then, was clear: “We have to conclude there is liquid water on Mars.”
The depth of the lake — the distance from top to bottom — was impossible to measure, though the team concluded it was at least one meter and perhaps in the tens of meters.
Might the lake be a habitable? Orosei said that because of the high salt levels “this is not a very pleasant environment for life.”
But who knows? As he pointed out, Lake Vostok and other subglacial Antarctic lake, are known to be home to single-cell organisms that not only survive in their very salty world, but use the salt as part of their essential metabolism.
(This column was written by my colleague Elizabeth Tasker, now at the Japan Aerospace Exploration Agency (JAXA), Institute of Space and Astronautical Sciences (ISAS). Trained as an astrophysicist, she researches planet and galaxy formation and also writes on space science topics. Her book, “The Planet Factory,” came out last year.)
June 30th has been designated “Asteroid Day” to promote awareness of these small members of our solar system. But while asteroids are often discussed in the context of the risk they might pose to the Earth, their chewed up remains around other stars may also reveal the fate of our solar system.
It is 6.5 billion years into our future. The sun has fused hydrogen into a core of heavier helium. Compressed by its own gravity, the helium core releases heat and the sun begins to swell. It is the end of our star’s life, but what will happen to the solar system?
While very massive stars end their element-fusing days in a colossal explosion known as a supernovae, the majority of stars in our galaxy will take a less dramatic exit.
Our sun’s helium core will fuse to form carbon but there is not enough mass to achieve the crushing compression needed for the creation of heavier elements. Instead, the outer layers of the dying star will be blown away to leave a dense remnant with half the mass of our current sun, but squeezed down to the size of the Earth. This is a white dwarf; the most common of all stellar ends.
The white dwarf rapidly cools to become a dim twinkle in the sky. Within a few million years, our white dwarf will be less luminous that the sun today. Within 100 million years, it will be dimmer by a factor of 100. But examination of white dwarfs in our galaxy reveals this gentle dimming of the lights is not as peaceful as first appears.
Infrared observations of over forty white dwarfs have additionally revealed compact dusty discs circling the dead stars. Sitting within the radius of a regular star, these could not have formed before the star shrank into a white dwarf. These must be the remains of what occurred as the star morphed from a regular fusion burner into a white dwarf.
This grizzly tale begins with the star’s expansion. Inflated by the heat from the helium core, our sun will increase to 230 times its current size. The outer layers will cool to emit a red hue that earns this bloated dying star the name “red giant”.
The outer layers of our red giant will sweep outwards and engulf Mercury and Venus, possibly stopping just short of the Earth’s position. But for any life remaining on our planet’s surface, the difference between envelopment and near-envelopment is rather moot.
The sun’s luminosity will peak at about 4000 times its current value, roasting Mars and triggering a whole new set of chemical reactions in Jupiter’s huge atmosphere. As the outer layers blow away and the red giant shrinks in mass, the surviving planets will drift outwards onto longer orbits, circling the white dwarf remnant at around twice their current distance from the sun.
But if the surviving planets are pushed outwards and the innermost worlds engulfed and vaporized, what is the origin of the compact disc and rocky pollutants? The answer, explains Dimitri Veras, explains Dimitri Veras, a planetary scientist at the University of Warwick in the UK, is asteroids.
Sitting between Mars and Jupiter, the asteroid belt is a band of rocky rubble left over from the planet formation process.
Occasionally, a kick from Jupiter’s gravity can send these space rocks skittering towards the Earth. These become known as “Near-Earth Objects” (NEOs) and are studied both for the potential threat to our planet should they collide, and also for their scientific value as time capsules from the earliest stages of planet formation.
At the moment, two missions are en-route to bring a sample from two different asteroids back to Earth. Japan’s Hayabusa2 mission has just arrived at asteroid Ryugu, returning stunning images of the asteroid to Earth. The NASA OSIRIS-REx mission is traveling to asteroid Bennu, and will arrive later this year.
But sitting further out than Mars, should not the majority of these small celestial bodies be unaffected by the sun’s demise? The problem turns out to be radiation.
Walk outside on a sunny afternoon and you are likely to notice that the ground beneath your feet is hottest at around 2pm in the afternoon, several hours after the sun has moved from directly overhead. This is because it takes time for the pavement to warm and re-emit the solar radiation as heat.
During that time, the Earth has rotated so that this heat radiation is released in a different direction to the absorbed radiation. Like catching a ball and throwing it away at an angle, this difference in direction gives the planet a small kick.
This kick is too small to make a difference to the Earth, but it can have a much more significant result on the evolution of an asteroid. The result is known as the YORP effect (standing for the Yarkovsky-O’Keefe-Radviesvki-Paddock effect, after the mouthful of researchers who developed the theory) and the related phenomenon named after the same first researcher, the Yarkovsky effect. Stemming from the push due to the uneven absorption and emission of radiation, the YORP effect causes a turning torque on asymmetric bodies while the Yarkovsky effect results in a push.
As radiation absorption and emission depends on the individual asteroid’s composition and topology, these forces are immensely hard to predict. This point was driven home in February 2013, when the world was primed for the close approach of asteroid Duende.
While everyone watched the sky in one direction, a second asteroid shot towards the Earth and exploded above Russia. This was the Chelyabinsk meteorite whose collisional path had not been anticipated. Studying the changes in an asteroid’s path due to radiation is therefore one of the primary goals of the OSIRIS-REx mission.
Given these challenges at the sun’s current level of radiation, it perhaps is not surprising that the red giant phase has more violent consequences.
Too small for gravity to pull them into a sphere, asteroids are typically lumpy rocks resembling potatoes or dumplings, like the rocky destination of Hayabusa2 and its predecessor which visited asteroid Itokawa. This asymmetry causes differences in the radiative force across the asteroid and creates a torque. This is the YORP effect and it spins the asteroid. As these small bodies typically have a weak tensile strength, the asteroid can self-destruct by spinning itself to pieces.
As the radiation from our swollen red giant beats down on the asteroid belt, these space rocks will start to spin and fission. The pieces will form a disc of dust around the dying star as it becomes a white dwarf, slowly accreting onto the dead remnant to pollute its atmosphere .
So is this now the end of our tale? A white dwarf surrounded by the fissioned remains of the asteroid belt, orbited by our more distant planets on wide orbits? It could be, depending on the existence of Planet 9.
Proposed by Mike Brown and Konstantin Batygin at the California Institute of Technology, Planet 9 is a possible addition to our solar system that sits on a very distant orbit beyond Neptune. Its presence is suggested by the alignment of six small objects in the Kuiper belt, a second outer band of rocky rubble that includes the dwarf planet, Pluto.
How Planet 9 might have formed remains a subject of debate. A likely scenario is that the planet formed in the neighborhood of the gas giants, but was thrown outwards in a game of gravitational pinball during a chaotic period as our planet-forming disc was evaporating. If this is true, the planet may be able to enact a terrible revenge.
Running a set of 300 simulations, Veras discovered that the fate of Planet 9 will depend on the planet mass, the distance of its current orbit and how rapidly the sun loses its mass. In the most benign outcome, Planet 9 meets the same fate as the gas giants and drifts outwards onto an even longer orbit. However, there are two situations in which this expansion causes the orbit of Planet 9 to bend.
If a star loses mass gradually, then the orbiting planets will gently spiral outwards and keep their nearly circular paths. But if the stellar mass loss is more rapid, then very distant planets that are more loosely held by the star’s gravity may undergo a runaway expansion of their orbits. As the planet shoots away, its orbit can become bent into an ellipse.
Such distant worlds also risk becoming susceptible to the gravitational tug of the surrounding stars in the galaxy. Known as the “galactic tide”, this force is much too weak to affect the planets in their current positions. Yet if Planet 9 drifts too far outwards, then the tidal forces could become strong enough to bend the planet’s orbit.
On an elliptical path, Planet 9 could move from its distant location to swing into the neighborhood of the gas giants. If the planet is massive enough, this could result in either Uranus or Neptune being ejected from the solar system to become rogue worlds: a fitting, final revenge for Planet 9.
Veras’s calculations suggest the most risky discovery for internal harmony would be a Jupiter-sized Planet 9 on an orbit beyond 300 AU, or 300 times the current distance between the Earth and the sun. For comparison, Neptune sits at 30 AU and the dwarf planet Sedna is three times as far, at about 86 AU. Alternatively, a smaller super-Earth Planet 9 could pose a risk if it was further out than 3000 AU.
Observing the gory remains of this process in other star systems provides us with more than just an eerie snapshot of our future. The crushed up asteroids in the atmosphere of white dwarfs reveal the composition of that planetary system.
“There’s no other way of performing an exoplanet autopsy,” explains Veras.
The results can reveal whether the asteroids and planets that orbited the star have a similar composition to our own or something more exotic. So-called “carbon worlds” have been proposed to orbit stars more carbon-rich than our own, whose rocky base may contain graphite and diamond rather than silicates.
So far, the planet autopsy has shown Earth-like remnants, but this is one area in which we would love to see more dead remains.
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