The Interstellar Neighborhood

This chapter discusses possible destinations for low gamma spacecraft. There are a number of G-class stars accessible to low gamma spacecraft. These might have terrestrial planets worth exploration. Most of the nearby stars are M-class red dwarf stars. It is unlikely that biologically active planets exist around these stars. However, since they make up the largest number of stars in the galaxy they are still systems worth a close examination, possibly by a starcraft.

The stars within 10 light years from the sun are listed below

Star name

Distance ly

Type

Msol

Planet

a Centauri 3

4.36

G2, K0-1, M5.5

1.1, .5, .12

0

Barnard's Star

6.0

M 3.8

.17

0

Wolf 359

7.8

M

~ .1

0

Lalande 21185

8.3

M 2.1

.46

3

Sirius 2

8.6

A0-1, DA2-5

2.14, 1.0

0

Luyten 726-8AB

8.7

M5.6, M6.0

.10

2?

Ross 154

9.7

M3.5

.17

Multiple entries for the star type indicate that the star system consists of two or more stars in a gravitationally bound system. The D-class star associated with Sirius is a white dwarf star, which is the collapsed core of a star. Most of these stars are red dwarf stars. The only star in this immediate neighborhood with identified planets is a dwarf star. It is plausible that such stars might have terrestrial planets, maybe life bearing planets. Though it is likely this is overly speculative, for such a planet may well be tidally locked in the same manner the moon is locked to show one face towards the Earth. Thus one face of the planet would be hot while the other frigid. So we may have to explore further out in order to find star systems that might offer a greater prospect for life. However, even within this small interstellar radius if it is possible to explore all of these stars there might be much to be gained. It is likely that dwarf star systems carry secrets of stellar evolution that warrant close examination. So they should not be excluded from any possible future plan of stellar exploration. However, the most exciting discoveries are likely to come from stars more similar to our sun. Figure 12.1 illustrates the mass distribution of stars.

Fig. 12.1. Histogram of stellar masses within 10 parsecs of Earth.

The detection of extrasolar planets had its start with the finding of a gas giant around 51 Pegasus [12.1]. This gas giant is close to the star and its gravitational tugging on this planet more pronounced. Consequently, the star wobbles about the mutual center of gravity for the star and the gas giant. There is a Doppler shift in the spectra of light from the star. Such "torch" gas giants are comparatively easy to find as a result. Gas giants further from the parent star exert a much weaker pull on their star, and the frequency with which the star wobbles is much lower. Jupiter orbits the sun every 11.8 years, which defines the periodicity with which the sun would be observed to Doppler shift according to its wobble. Kepler's law also predicts a much smaller velocity for the sun by v = ur = r. Jupiter would be a more difficult gas giant for some extra terrestrial astronomer to find than the 51 Pegasus gas giant.

Diagram Vega
Fig. 12.2. The Hertzsprung-Russell classification chart for stars.

Stars are classified according to something called the HertzsprungRussell diagram. This is an empirical chart that relates the luminosity of a star to its surface temperature. From the most luminous to the least the sequence runs as OBAFGKM, with the neumonic "Oh Be A Find Girl Kiss Me." The chart illustrates a definite trend where the higher the luminosity the higher the temperature, where this main band of stars define the main sequence stars. The diagonal lines indicate the diameter of these stars, where an M class star with a large radius is a red giant. There is also a domain for white dwarf stars. It is also clear that on the main sequence there are far more red dwarf stars than there are whiter and hotter stars, such as the sun. It is common to say that the sun is a rather common star, when in fact the most common stars are red dwarf stars. A planet that is biologically active is most likely, at least based upon our local experience, to exist around an F to K class star. The sun is a G-class star which is in what might be the sweet spot for life bearing planets. Higher luminosity stars planet are less apparent, and further these stars have short lives as they consume their nuclear fuel rapidly. So life is very unlikely to exist around any star in the O through A domain. Biological evolution requires lots of time. The situation might be more forgiving for smaller stars that fall into the lower K to upper M range. Of course for such stars with weaker luminosities the orbit of the planet must be much closer to the star. This orbit cannot be so close as to cause the planet to be tidally locked to the star, just as our moon always presents one face to the Earth.

The number of stars that exist within an increasing radius of space within the spiral arms of the galaxy grows very quickly as the cube of the radius. So below is a chart of stars that fit within the F to K range of the Hertzsprung-Russell diagram. For starcraft sent to further destinations these would most likely be the destination of choice. Some of these stars have planets or brown dwarfs that have been identified by their Doppler shift induced by gravitational wobbling. This and other charts below list these stars by name, distance in light years, their mass in solar mass units and how many gas giants or brown dwarfs have been identified. A chart of the nearest stars is:

Name Distance Type Mass Planet e Eridani 10.52 K2V .85 1

These stars are potentially accessible by a photon sail craft. The star e Eridani is a fairly young star, < 109 year old, with a planetary disk and one detected planet. So this may reflect a solar system similar to ours in its early phase of development. Procyon is more problematic for containing a biologically active planet. The star has no identified gas giant planets so far. Further, this star has a white dwarf companion in orbit about it, which would be situated around the orbit of Saturn. This would likely be too large a gravitational perturbation for any stable solar system of planets. Procyon is also likely to be too energetic, some 7.5 the luminosity of the sun, for any possible planet with stable chemistry for the emergence of life. e Indi is an orange dwarf star similar to e Eridiani. It further has a brown dwarf star that orbits around it with a mass of .002 the mass of the sun. This is 43 times the mass of Jupiter, and this companion orbits the star at a distance of 1500 astronomical units, or 1500 times the distance from the Earth to the sun. The hunt for planetary bodies closer to this star has been inconclusive. t Ceti is a star similar to the sun, but is deficient in elements beyond hydrogen and no planets have been discovered. t Ceti has a dust cloud around it, which suggests asteroids and comets. If t Ceti has a gas giant it is likely modest in mass and in a large orbit. This star is often the home of ETI in science fiction, but the prospects for a terrestrial planet appears at best uncertain.

Below are F through K stars within a 20 light year distance. Most of them are K-class stars with no appearance of gas giant planets. However, a modest gas giant in a distant orbit could well exist around some of these stars. This list is below

Name

Distance

Type

Mass

Planet

Lacaille 8760

12.9

K7

.6

0

Groombridge 1618

15.9

K7Vne

.64

0

40 Eridani

16.5

K1Ve, DA4, M4.5Ve

.89, 51, .2

0

70 Ophiuchi

16.6

K0-1Ve, K5-6Ve

.92, .7

0

a Draconis

18.8

K0V

.89

0

n Cassiopeiae

19.4

G3V, K7V

.9 - 1.1, .6

0

J. Herschel 5173

19.7

K2, M4

8 , 2

0

82 Eridani

19.8

G5-8V

.97

0

S Pavonis

19.9

G5-8V

1.1

A brief description of these stars is given by:

• Lacaille 8760 is a K-class star with no identified gas giant planet. It is also a flare star, with erratic eruptions of energy. This might prevent any bio-active terrestrial planet within the habitable zone of ~ .16AU.

• Groombridge 1618 is a K-class star with some similarities to the sun. However, no Jovian planet greater than seven times the mass of Jupiter have been identified.

• 40 Eridani is an orange K-class star with both a white dwarf and a red dwarf companion. So far no planetary bodies have been identified. The white dwarf star is thought to be relatively young. This means that in recent history it blew off its outer layers, which would was a violent event for any possible stellar system of planets. This likely negates any prospect of a biologically active planet.

• 70 Ophiuchi is a double star system of two K-class stars separated by 23 AU. Such a double star may not be conducive for planets, and so far none have been found.

• a Draconis is a bright K-class star that is somewhat poorer in heavy elements than the sun. So far no gas giant planets have been found.

• n Cassiopeiae is a double star, consisting of a gravitational bounded orbit of a K-class star and a G-class star. These two stars are 23AU in separation, or separated by a distance equal between the orbit of Neptune and Pluto. No Jovian class planets have been identified. The G-class star contains about two thirds the abundance of elements heavier than hydrogen relative to the sun.

• J. Herschel 5173 is a K-class star with a red dwarf companion in a 43AU distant orbit. The K-class star is as abundant in heavier elements as the sun. However, no planets have been detected so far.

• 82 Eridani as a G-class star has almost ideal spectral characteristics for a life bearing planet, However, no gas giant planets have been identified. It also is lower in heavier elements than the sun, which makes it less probable that terrestrial planets exist. It also appears to be drifting off the main sequence and evolving towards its red giant stage of evolution.

• S Pavonis is almost identical to 82 Eridani. It also appears to be aged and near the end of its main sequence stage of stellar evolution.

This appears to present a paucity of star systems that might bear life. However, none have torched gas giants that could make the orbits of terrestrial planets far more unstable over the duration of the stellar system. Some of these might have modest gas giant planets in large orbits as yet unidentified. If we push further out there are more star candidates to examine. Below is listed only the G-class stars

Name

Distance

Type

Mass

Planet

£ Bootes

22.1

G8Ve, K4-5Ve

.9-.94, .7

0

H Cassiopeiae

24.6

G5VI

.6-. 8

0

£ Ursae Majoris

27.2

GOV, G5IV

1.05, .9

4

Chara

27.3

G0V

1.1

0

H Herculis

27.4

G5IV

1.1

0

61 Virginis

27.8

G5V

.96

1

ß Comae Berenices

29.9

G0V

1.05

0

Groombridge 1830

29.9

G8VI

.6

0

k Ceti

29.9

G5V

1.0

These stars are described as:

• £ Bootes is a double star, a G-class and K-class star in a gravitational system. The G-class star is comparatively poor in heavy metals compared to the sun. So far no Jovian type of planet has been detected. The

K-class companion star is rich in heavy elements, which might reflect the environment around the main star. The two stars orbit an average of 33.6AU, which might make stable planetary orbits problematic. However, this is a plausible candidate for terrestrial planets

• ¡i Cassiopeiae is a small G-class star that is poor in heavier elements. It also has a red dwarf companion. This star is likely to be a halo star as indicated by its velocity. No planets have been found around this star and is a fair to poor candidate for terrestrial planets.

• £ Ursae Majoris is an interesting double star where both are G-class stars. There are 3 brown dwarf stars identified. This system is also rich in heavier elements. This star system may, unless gravitational perturbations prevent it, be a decent candidate for a search for terrestrial planets. This is a possible candidate for a biologically active planet and a possible destination for an interstellar probe.

• Chara is a metal rich star similar to the sun. While as yet no Jovian planets have been identified around this star it is a decent candidate for a terrestrial planet.

• n Herculis is an older version of the sun. This star has three red dwarf companions in close orbit, This may make the environment too unstable for there to be planets or terrestrial planets.

• 61 Virginis is one of the best candidates for a search for an terrestrial planet. A 20 times Jovian mass planet has been found, which indicates an active solar system. Further, the star is about as rich in heavy elements as the sun.

• ¡3 Comae Berenices is a slightly larger version of the sun. It is also likely to be younger than the sun. So far no gas giants have been found around this star. It is also about twice as abundant in heavy elements. This star may then be a good candidate for a terrestrial planet.

• Groombridge 1830 is a smaller and low luminous G-class star that is very deficient in heavy elements. This makes this star a poor candidate for terrestrial planets.

• k Ceti is a young G-class star that is rich in heavy elements. It would be a fair candidate for a biologically active planet, one early on its course of evolution, but the star is subject to violent flaring. This likely makes life on any planet impossible.

Very recently a planet about 5 times the mass of Earth with 1.5 times

Earth's diameter has been found around the red dwarf star, spectral type

M2.5V, Gliese 581 [12.2]. This planet is in the mass range for a terrestrial or rocky planet, and is not likely to be a gas planet. The planet is close enough to this star to have a temperature of 0-40°C, which will support liquid water if the planet has an atmosphere. Because of the mass and dimensions of the planet the gravitational acceleration is 2.2 times that on Earth. This planet is close to the star, .067 AU, and since the star is about 1/3 the mass of the sun this planet orbits this star every 13 Earth days.

Tidal accelerations are ~ d/r3, for d the diameter of the planet and r the radial distance to the large body. The ratio of the tidal force on the moon to that on this planet is

Fmoon mmoon dmoon mearth ( rpl r\

Fpl mpl dpl mstar ^rmoon which is about 7.78 x 10~7, for the subscript pl pertaining to the Gliese 581 planet, and the number 1.5 pertains to the ratio of the mass of the planet to the Earth. The radial acceleration that would spin down the planet to a tidal locked rotation would then be about 1.05 x 102 times that which locked the moon. Over billions of years the effect may well slow down the rotation of the planet so it could be tidally locked. If this is so it might make the planet less habitable than presumed, where one face is baked and the other cold. Such a planet might have a belt of life that extends around the limb of the planet as seen from the star, with deserts at the antipodal regions facing towards and away from the star. Life would then exist in a extraterrestrial geographic regions where this star would appear near the horizon.

There is an approximate formula for the time it takes a satellite to become tidally locked. For a satellite of mass m in orbit around a large body with a mass M at a semimajor axis radius a the time it takes for tidal locking is [12.3]

ma61 w Q

Mock- 3GM2R5 x k-

Here w is the initial angular velocity of the satellite. I — .4mR2 the moment of inertia for a spherical satellite, and G the gravitational constant. The constants Q and k are the disspation function and tidal Love number of the satellite. These are generally less certain, but for the moon the ratio of the two is Q/k — 910. By inserting numbers for this planet the time for tidal locking is then given by

If this planet is assumed to intially rotate every 24 Earth hours it would then take about 2.16 x 1012s, or 6.87 x 104 years, for the planet to spin down into a tidal lock. This clearly indicates that the planet is tidally locked. Certainly now, billions of years after the formation of this stellar system, this planet is tidally locked. This planet was most likely tidally locked within a few hundred thousands of years after its formation. This does not preclude the existence of life, but it would likely mean that life is settled along some zone on the planet where the star Gliese 581 would be seen near the horizon, or just off the horizon.

Since the star is an M class star there is little blue light, so during day the sky would probably be black. I would make the supposition that since this planet has a hot side and a cold side, and if it has an atmosphere, this would mean that these thermal gradients might drive horrendous storms in this zone. The temperature in this goldilocks zone might be comparable to what we have on Earth, but gravity and climate could well make this planet uncomfortable. From the perspective of our comfortable planet Earth this planet is likely a horrid place, at least for us. Even if there is life in this zone, it is likely no more evolved than unicellular forms. It might exist in prokaryotic mats, similar to what the early Earth had as seen with stromatolites. Of course it is difficult know what this planet might be like. A look at early ideas about what Venus might have been like to see that such ideas are too often wrong. Yet if there is water on this planet it might be that a lot of it is in the vapor form due to the hot side. If there is too much water the whole planet might be enshrouded with clouds, and with carbon dioxide this planet might be similar to Venus, but with water vapor in the atmosphere. It could well be a very hot sauna of a planet enshrouded by heat trapping clouds. A close view of the planet by an optical interferometer is the only way coarse grained information about this planet can be obtained. To get a close up view of its geology and the remote prospect for life will require that a robotic probe be deployed on the planet.

The astronomers used the HARP instrument on the European Southern Observatory 3.6 meter telescope in La Sille, Chile to detect this planet. They employed the Doppler shift due to radial velocity of the star, or "wobble," technique. These data are used with Newton's laws to determine the size and mass of a planet, based on small wobble perturbations of the stellar motion off its center by gravitational pull of the planet. The planet is only 20.5 light years away, and so optical interferomters might be able to get a fair look at it. It is further well within the range of a starprobe, and potentially accessible to a photon sail craft.

In the survey of the local stellar neighborhood stars comparable to the sun are mostly featured, while red dwarf stars are largely ignored. This is because M spectral type stars are very numerous, and secondly they are dark horse candidates for biologically active planets. However, they may be of interest in their own right, and their sheer numbers might by the rule of statistics make their planetary systems highly diverse in structure.

The number of G-class stars of course increases the further out one looks. However, this list is cut off at 30 light years, for at best it will take 75 to 100 years from mission launch to the receipt of information from these stars. To reach these 20-30 light year distances and beyond the mission is similar to the Cathedral building in the late middle ages, which took the better part of a century or more to complete. The originators of them never saw their completion. Certainly to probe further out into interstellar space will require such multi-generation efforts. Yet even with the sketchy information that exists there is tantalizing prospects for terrestrial planets. Further, if life is relatively abundant in the universe one of these stars just might have an Earth-like planet. If one is identified by the optical interferometry there will be a scientific value in launching an interstellar probe to examine this planet.

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