Star system without planets


By Tom Burns - Stargazing



With all the talk about newly discovered planets orbiting stars apart from our sun, it’s easy to assume that planets are common as dirt in the universe. However, one common starry characteristic makes planetary formation less likely than many folks might expect.

Our sun, which is, of course, loaded with planets, is unusual because it is alone. Most stars in the Milky Way galaxy have other stars orbiting them. Such systems are called “binary stars.”

Many stars exist in collections of more stars than binaries. Systems with three, or four, or more stars abound.

Stars are enormously massive hydrogen bombs. The presence of another star with its mass-generated gravitational pull makes planetary formation difficult — but not impossible.

If, against the odds, a planet forms around one of the stars, the presence of the second star can add instability to the planet’s orbit, and that spells doom to the formation of life.

A classic example of the first problem is visible right now in an amateur telescope. Look toward the southwest for the Y-shaped constellation Virgo, the Maiden. At the split of the “Y” is a yellowish star easily visible to the unaided eye.

That star, called Porrima, gets its name from a Roman goddess of prophecy and childbirth.

A telescope at high magnification reveals that the star is, in fact, two stars — pale yellow beauties that look like twin headlights coming right at you from the inky depths of space.

Porrima is a genuine, gravitationally-linked binary star around 35 light-years from Earth. (One light-year is equivalent to about six trillion miles.)

Its twin stars are similar, relatively speaking, to our daystar, the sun, at about 1.5 times its mass.

Since the two stars have about equal mass, it’s difficult to say which star is orbiting the other. In a real sense, they are orbiting each other like your thumbs twiddling, except that one twiddle (orbit) takes about 170 years. However, if you were standing on one of the stars (wear thick-soled boots!), it would undoubtedly look like the other star was orbiting you, so we’ll look at things from that perspective.

As far as planets are concerned, the problem with this “double star” is that the orbit of one star around another is rarely circular. The orbits are almost always highly elliptical and thus very stretched out.

In the case of Porrima, when the stars are at their widest apart, they are 38 astronomical units, or AU’s, apart. One AU is the distance from our Earth to the sun, so Star One is somewhat farther from Star 2 than Pluto is from the sun.

Unfortunately, for about 85 years, Star One crashes down toward Star Two until they are a scant 3 AUs from each other. That’s just twice the distance that Mars is from our sun — too close for gravitational comfort, to say the least.

The stars were last at maximum separation late in 1920, and observers using even the tiniest telescope could see them as two stars. They approached each other for the next 85 years, and the pair became increasingly difficult to split in an amateur telescope.

By 2002, you’ll have to trade your small refracting telescope for a larger amateur instrument.

In 2006, they reached their closest point, called periastron by astronomers. At that point, you would have needed a large amateur telescope and a night free of atmospheric turbulence to split them.

After 2007, the stars slowly widened their distance. In 2010, they were just getting to the point where they might have split in a large amateur telescope.

By now, the stellar twins are again splittable in most amateur telescopes. In 2090, they will reach apastron, their most-distant separation.

Imagine a giant hydrogen-bomb star orbiting the sun to understand the effect on planetary formation. Every 85 years, it slowly dives toward Earth. Add its energy to the sun’s already prodigious output, and one conclusion is unavoidable.

The combined energy output would fry our planet like a giant chicken nugget. Because of the extreme variations in temperature, life would never have formed on Earth in the first place.

Indeed, the planets would never have formed. Our solar system coalesced from a swirling disk of dust and gas. Slowly, over many millions of years, smaller hunks of material were attracted to each other by their mutual gravity. As they smashed together, they heated up and formed liquid balls of rock, metal, and gas that cooled to create lovely spherical planets.

The planet-killing capability of a second star doesn’t arise out of its temperature. It comes from its massive gravity, which would drag planet-formed materials out of the potential solar system and stir things up so severely that planets could never form.

In a way, we owe our lives to our sun’s unusual aloneness in the cosmos.

Planets are undoubtedly possible in two-star systems, but the two stars must orbit each other at a great distance. Alternatively, one of the stars can’t be very massive.

Such is the case of the star marked 70 Virginis on most star maps.

The star’s mass is similar to the sun’s 1.1 solar masses. It has reached an advanced age of almost eight billion years and swelled to twice the sun’s diameter. Despite its similar mass, it produces nearly three times the sun’s energy output.

70 Virginis has a companion star, but the companion did not prevent the formation of at least one planet. Its companion, which has only eight percent of the sun’s mass, is barely a star.

The main star has at least one planet of a class called a “hot Jupiter,” a gas giant planet orbiting very close to its star. It is at least seven times the mass of Jupiter and orbits so close to the star that it completes one orbit every 111 days.

The sun’s planet Mercury completes an orbit every 88 days. It’s hard to imagine life forming in such a harsh environment.

If our sun is any indication, the best chance for forming planets with life on them occurs when a star has no stellar companion. Such is the case with 61 Virginis.

The star is relatively close to us at 28 or so light-years away. Its diameter and mass are similar to the sun, and it produces about 80 percent of the sun’s energy. It appears to be only a billion years older than the sun.

And it has planets! Here the comparison ends, however. Its three worlds are not very promising as far as life is concerned.

The first planet in orbit around 61 Virginis has a mass of about five times that of Earth. NASA publicists call such planets “super-Earths,” which sounds a lot better than it is.

The planet practically skirts the star’s surface at only 4,650,000 miles away. By comparison, Mercury has a surface temperature of 600 degrees Fahrenheit and is 33 million miles from the sun.

The second planet isn’t much better off. Its mass is 18 times that of Earth’s. Its short orbital period of 38 days puts it only 18,600,000 miles from the star. Planet Two is still inside the close orbit of Mercury, comparatively speaking.

The third planet fares a bit better. It’s massive, to be sure, at 25 times that of Earth’s mass. However, its orbital period of 123 days puts it 44,300,000 miles from the star — outside the orbit of Mercury but well inside the orbit of Venus.

The possibility remains that we will discover Earth-like planets orbiting Virgo’s stars at the correct distance to sustain life. After all, the techniques for finding such planets are still in their infancy. However, one thing is undeniable.

Numerous elements must come together before planets suitable for life occurs. We have discussed only a few of them here. Humans have come to call such a convergence of many elements “luck.”

As you stare at the stars of Virgo, consider how extraordinarily lucky you are to be on spaceship Earth — orbiting a suitable star on a suitable planet at a convenient time as we all rush through the unforgiving depths of space.

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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.