Closer look at Proxima’s planets


By Tom Burns - Stargazing



Our sun is a star, by some measures an average one.

Old Sol, as our daystar is sometimes called, has reached a nice, respectable middle age at about 5 billion years old. In five or six billion years, it will run out of fuel and its thermonuclear fire will abruptly end. Then, the sun will collapse into a tiny white dwarf just a few thousand miles wide. Such is the fate of “average” stars like our sun.

However, when astronomers examine the stars nearest to our sun and Earth, they discover that 20 out of the 30 nearest stars are red-dwarf stars, which are not much like our sun at all.

Like our sun, red dwarfs are hydrogen bombs that fuse their hydrogen into helium and produce vast quantities of energy.

Red dwarfs, which make up perhaps 70% of the stars in the universe, distinguish themselves because they are very low-mass balls of hydrogen and helium. At 10-20% of the mass of the sun, they simply lack the “star stuff” that characterizes other categories of stars.

As a result, they fuse their hydrogen much more slowly. Because they lack the large quantity of hydrogen fuel possessed by a medium-sized star like our sun, they last a lot longer. Our sun will maintain its hydrogen-bomb reaction for 10 or 11 billion years. A typical red dwarf sustains its thermonuclear furnace for trillions of years.

The nearest star to our sun, called Proxima Centauri, also happens to be a red dwarf. As a result, Proxima is a prime object of study by astronomers and is therefore one of the best-known examples of that category of stars.

Proxima has about an eighth the star stuff of our sun and about one-seventh of our sun’s million-mile width. Consequently, it may continue to fuse hydrogen at a more-or-less steady pace for four trillion years.

Such extreme longevity and relative stability are two of the many conditions that make the development of life possible. If planets form around a red dwarf, they will be the recipients of a steady stream of energy for a very long time. If other conditions are right and life forms, that life could potentially have a very long time to prosper and develop.

As a result, red-dwarf stars are prime candidates for the search for Earth-like planets and the life they might harbor.

On Jan. 15, a team of astronomers announced the discovery of a planet, dubbed Proxima c, orbiting Proxima. The planet is apparently a “super-Earth” about six times the mass of our planet. It orbits Proxima every 5.2 Earth years with a distance of about 1.5 times the distance Earth is from the sun.

Given Proxima’s low-energy output, Proxima c must be pretty frigid by Earth standards. At -388 degrees Fahrenheit, Proxima c never gets warm enough to have liquid water on its surface, and liquid water is a prime ingredient for life. The planet is thus said to be too far outside the habitable zone of the star, a region that astronomers have dubbed the Goldilocks Zone.

As a result, Proxima c is no big deal, except for one thing. It’s not the first planet discovered orbiting around Proxima. That makes Proxima and its planets a stellar system like our solar system.

As it turns out, such systems of multiple planets are relatively common among nearby red-dwarf stars. Trappist 1 in the constellation Aquarius, is the best case so far. Trappist has drawn the attention of astronomers because of its seven Earth-sized planets, at least three of which are within the Goldilocks Zone.

So how do we find out if life is present on an Earth-like planet? The surest way is to go there.

Unfortunately, at 40 light-years away, Trappist 1 is far beyond the range of current space-flight technology unless we’re patient enough to wait hundreds of years for up-close data.

However, the Proxima system is only 4.2 light-years away. Plans are currently underway to send a probe to it. If that probe can overcome the myriad dangers of a velocity approaching 20% of the speed of light, an unmanned spacecraft could reach the Proxima system in “only” 25 years.

Thus, even if Proxima c is something of a bust as far as potential life is concerned, Proxima’s other planet turns out to be far more promising.

In 2016, an international team of scientists led by Guillem Anglada-Escudé from London’s Queen Mary University, announced the discovery of Proxima b, an Earth-sized planet orbiting Proxima. Their announcement in the journal Nature was startling for more reasons than the planet’s size and proximity to Earth. The planet is also orbiting within the star’s Goldilocks Zone.

For a cool red dwarf like Proxima, planets in the habitable zone must be much closer to the star than Earth is from our sun. Earth is 93 million miles from its star. Proxima’s planet huddles up near Proxima at only 4 million miles away and zips around the star in only 11 days.

You won’t find any images of the planet because astronomers have not seen it visually. Its presence was detected by looking for a wobble in the motion of Proxima. The star’s wobble suggests that an object at least 1.3 times bigger than Earth is whipping around Proxima.

Astronomers haven’t visually observed the wobble either. It was detected by looking at changes in the star’s rainbow band of light, called its spectrum. That “Doppler method,” as it is called, turns out to be a pretty reliable way of discovering objects orbiting distant stars. Scores of planets have been discovered using it.

Because of Proxima b’s Earth-like status, some writers enthusiastically touted the possibility of life on the planet. However, most astronomers weigh in with healthy skepticism. As far as we know, a planet needs to fulfill many conditions besides size and distance to create and sustain life. Let’s see if Proxima b fulfills some of those criteria.

First, life requires a stable star. It’s hard to argue with Proxima’s stability over the long haul. Four trillion years is a long time to be fusing hydrogen. However, over the short run, Proxima has its problems. Like most red dwarfs, Proxima is a flare star. Every few hours, it increases dramatically in brightness and energy output. Those flares produce a large surge of deadly X-rays.

During a typical flare, Proxima’s energy output increases by only 20%, which is not that bad. In fact, during a flare, Proxima produces only the amount of radiation that our much larger sun produces all the time.

However, the planet’s proximity to Proxima factors into the equation. At a scant four million miles away from its parent star, Proxima’s planet probably receives lethal doses of X-rays during Proxima’s frequent flares.

Conceivably, the planet’s life could have adapted to such bursts of energy. However, it is less likely that life could have been created in the first place. If Proxima has some form of life, it was probably was born and survives deep underground.

Another requirement for life is a planetary atmosphere of just the right density and composition. Proxima’s energy output is so low that the planet’s surface is heated to only minus 40 degrees Fahrenheit. Its atmospheric blanket has to hold in enough heat to warm the planet sufficiently to keep water in liquid form, which is the fundamental definition of a habitable planet.

The trouble is, we will not find out if Proxima has the right kind of atmosphere — or, in fact, any atmosphere at all — for a very long time. The planet is simply too small and too close to its parent star to observe it directly.

Astronomers hope that, at some point, Proxima’s planet will pass across the face of the star from our vantage point on Earth. In that case, astronomers could potentially study the planet’s atmosphere by looking at the light from the star as it passes through its atmosphere.

However, that kind of research would require a telescope with extraordinary resolving power and of enormous size. Astronomers hope that the much-delayed Webb Space Telescope will begin to tell the tale of the planet’s atmosphere if, at last, the planet transits the star in the first place.

Adding to the difficulty again is the planet’s proximity to Proxima. Earth is far enough away from our sun to allow our planet to maintain its rotation. As our planet spins, the atmosphere sloshes around. That atmospheric motion redistributes the heat and allows a part of the planet to stay habitable.

Proxima b is so close to Proxima that it is almost certainly tidally locked to Proxima. One side is always facing the star, and the other side is locked in permanent darkness. The difference in temperature between the two sides must be very large indeed.

All things considered, we’ll probably have to send a spacecraft there to find out. Thus, it might be many decades before we discover whether Proxima’s planet holds even the most remote possibility for life.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.