When I was a teenager, I was a voracious reader of the work of the famous science fiction writer, Isaac Asimov. But it was not his science fiction I consumed – it was his essays on science fact. While I initially read his work on space and astronomy (virtually my only subjects of interest when I was younger), it was not long before I started reading his essays on physics, chemistry and other areas of science. I can credit Dr. Asimov with kindling my broader interest in all of the sciences and setting me down a path that led me to a career in physics.
Cover of the 1970s paperback edition of “Fact and Fancy” which contains Isaac Asimov’s essay, “The Planet of the Double Sun”.
I still recall one of Asimov’s essays I read over 35 years ago in one of the numerous paperback collections of his work I owned titled “The Planet of the Double Sun”. In this essay, Asimov wrote about how the sky would appear if instead of orbiting the Sun, the Earth orbited α Centuari A speculating how the changing position of the prominently bright α Centauri B over the course of the year and its 80-year orbit could have been connected to the myth about Prometheus by the ancient Greeks in such a circumstance. What stuck with me even after all this time was the thought of a sky with multiple suns and that Asimov was able to calculate how it would appear to an observer using fairly straightforward math.
In the normal course of going through the news on space and astronomy, I stumbled upon a paper by a team led by Lewis Roberts, Jr. (JPL) about their discovery of red dwarf companions orbiting two different stars previously known to host extrasolar planets. With this discovery, one of those stars, 30 Arietis (or simply 30 Ari), is now known to be a quadruple star system – the second such multiple star system known to host an extrasolar planet. What really piqued my interest was a quick two-sentence paragraph in a press release about the discovery from the University of Hawaii (the home institution for one of the new paper’s authors) about how these four stars would appear in the sky of this system’s known extrasolar planet. I immediately was reminded of the Isaac Asimov essay I had read so long ago. Since the short qualitative description in the press release lacked any specific numbers (and I knew the fanciful depiction by Karen Teramura that came with that press release shown at the top of this page was hardly accurate), I was curious about the details given the best information we have about this system and decided to do my own calculations.
Background
The 30 Ari star system is located about 135 light years away in the constellation of Aries. The two brighter components, known as 30 Ari A and B, have apparent V magnitudes of 6.48 and 7.09, respectively, with a separation of about 38 arc seconds. Even the blended images of this pair of stars is too dim to be seen with the naked eye from the Earth and requires at least some modest optical aid to spot. Studies of their motions indicates that these two stars are gravitationally bound with a separation of about 1,520 AU projected onto the sky (the actual separation is probably greater) and an orbital period estimated to be about 34,000 years. Comparison of the properties of these stars with models of stellar evolution indicate that they are about one billion years old, give or take several hundred million years.
The brighter component of the system, 30 Ari A, is a F5V star with 1.31 times the mass of the Sun, 1.37 times its radius and about 2.9 times its luminosity. This star is also known to be a single-line spectroscopic binary with a period of 1.11 days. Since the spectral lines of the companion have not been observed and the inclination of its orbit is unknown, it is not possible to determine the companion’s properties however, it is a pretty good bet that this star is a dim red dwarf orbiting only about 0.025 AU from its brighter and hotter sibling.
The third component of this system, and the second brightest in the system, is 30 Ari B. It is an F6V star with 1.11 times the mass of the Sun, 1.13 times its radius and about 2.0 times its luminosity. In 2009 Guenther et al. announced the discovery of a substellar companion orbiting 30 Ari B using precision radial velocity measurements. The companion they found, designated 30 Ari Bb, was in a 335-day orbit with an eccentricity of 0.29 and a mean orbital radius the same as that of the Earth. With a minimum mass or MPsini of 9.9 ±0.9 times that of Jupiter, this body is either a very massive gas giant or possibly a small brown dwarf. With a Venus-like effective stellar flux about twice that of the Earth, there is little prospect that any large moons of this giant world could be habitable. No other planets have been reported orbiting 30 Ari B in search results published to date.
The discovery of a fourth star in this system orbiting 30 Ari B, called 30 Ari C, was announced by Roberts et al.. To spot this new stellar companion, they used a series of images acquired in the i band (located on the red edge of the visible spectrum) using the Robo-AO imager on the 60-inch telescope at the Palomar Observatory and in KS band (in the near infrared) from the PHARO camera using the PALM-3000 adaptive optics system on Palomar’s famous 5-meter telescope. Roberts et al. spotted a new stellar companion that was 4.31 magnitudes dimmer in the i band than 30 Ari B only a half an arc second away. A comparison of images taken about a year apart in 2012 and 2013 shows that this newly found star shares the proper motion of 30 Ari B confirming that the two are physically associated with each other. With this discovery, 30 Ari is now known to be a quadruple star system. About 10% of wide binaries, upon closer inspection, turn out to be “2+2” quadruple star systems like 30 Ari.
The left panel is an i band image about 2.5 arc seconds across of 30 Ari B and C (the latter indicated by the arrow) from Robo-AO. The right panel is a Ks band image about two arc seconds across acquired using the PALM 3000 AO system. The faint object to the lower right of the primary star visible in the right panel is a ghost image caused by a reflection off of a neutral density filter. (Roberts et al.)
Based on their analysis, Roberts et al. estimate that 30 Ari B and C have a separation of 22.3 AU projected onto the sky with a statistically likely period within a factor of about three of about 80 years. This implies that the actual semimajor axis of the orbit of 30 Ari BC is probably within about a factor of two of the current apparent separation. Based on the photometry, it is estimated that 30 Ari C has a mass of about half that of the Sun making it a fairly substantial red dwarf probably with a couple of percent of the Sun’s luminosity. The presence of this stellar companion in a moderately distant orbit could help to explain the eccentric orbit of 30 Ari Bb.
The Sky of 30 Ari Bb
Given what is known about the 30 Ari system, I set about calculating how the various stars would appear from the vantage point of an observer near the only known planet in the system, 30 Ari Bb. Right away we have the problem that we do not know the actual orbits of the various stellar components of this system with respect to 30 Ari B. The motion of 30 Ari A and B, with a probable orbital period measured in tens of thousands of years, is too slow to have been reliably measured over the last few of centuries of telescopic observation and 30 Ari C has been observed for too short of a time. For my calculations, I just assumed that the actual separations of the various components in this system are equal to their current projected separations. This would yield a “best case” scenario for how these stars would appear today (i.e. it would maximize their brightnesses and apparent sizes in the configuration we observe today). In reality, the various orbits of these stars are likely to be moderately eccentric which would cause their appearance to change over time.
From a vantage point in the vicinity of 30 Ari Bb, the relatively nearby 30 Ari B would dominate the sky with a typical V magnitude of about -27.7 or twice the apparent brightness of the Sun as seen from the Earth. While the average apparent size of 30 Ari B would be only slightly larger than that of the Sun as viewed from the Earth at about 38 arc minutes, the higher effective temperature of 30 Ari B would make the surface brightness of this F-type star over half again higher. The eccentric orbit of 30 Ari Bb would also make the apparent size of its sun vary noticeably from 27 to 49 arc minutes during the course of its 335-day orbit. Likewise, its apparent brightness would range from 1.2 to 4.0 times the apparent brightness of the Sun as seen from the Earth over the course of an orbit. Such changes would be noticed even by a casual observer.
The next brightest star in the skies of 30 Ari Bb would be 30 Ari C. We do not have any visible band photometry yet for this newly discovered star but its V magnitude value is likely at least 5 magnitudes larger than that of 30 Ari B (in other words, less than 1/100th as bright). If we assume that 30 Ari C is 22.3 AU away, that translates to a V magnitude approaching -16 – over an order of magnitude brighter than a full Moon here on Earth. Given its low surface temperature, 30 Ari C would appear distinctly orange in color especially compared to the brilliant white of the two F-type stars that dominate the system. If 30 Ari C is typical of red dwarfs of its mass with a radius about half that of the Sun, it would present a disk around 40 arc seconds across as viewed from 30 Ari Bb. This is comparable to the apparent size of Jupiter as viewed from the Earth except 30 Ari C would appear several orders of magnitude brighter.
Because of the orbital motion of 30 Ari C, its position against the more distant background stars would change over time. Assuming a circular 22.3 AU orbit with a period of 80 years, the mean motion of 30 Ari C would be about 4½ degrees per Earth year – an amount that would be noticeable even to a casual observer. And if 30 Ari C is a typical red dwarf with its own compact system of planets orbiting within about a half an AU (see “Occurrence of Potentially Habitable Planets Around Red Dwarfs” and “Architecture of M-Dwarf Planetary Systems”), they could be observed moving as far as degree or so from this star over periods up to several months. The brightest and most distant of these planets could be potentially observable to the naked eye despite the brilliance of nearby 30 Ari C.
Even though it is much more distant, 30 Ari A would still present an incredible sight to an observer near 30 Ari Bb. With a V magnitude of -12.3, it would be almost as bright as a full Moon but compressed into a point of light instead of spread over half a degree of sky. A view through a telescope under high magnification (and plenty of neutral density filters!) would reveal a tiny, brilliant disk only about two arc seconds across. The closely orbiting and noticeably redder companion of 30 Ari A would appear perhaps five magnitudes dimmer than its brighter sibling never moving more than three or so arc seconds away as it scoots around 30 Ari A in under 27 hours. With an apparent magnitude of around -7, this “dim” companion would be very prominent in the sky if it were not orbiting so close to 30 Ari A. An especially keen observer looking through a 20-cm telescope or larger under excellent seeing conditions in an Earth-like atmosphere might even notice its tiny disk maybe around two-thirds of an arc second across.
Because of its huge orbit and long period, the orbital motion of 30 Ari A would not be very pronounced. Assuming a circular orbit with a radius of 1,520 AU and an orbital period of 34,000 years, 30 Ari A would appear to drift across the sky of 30 Ari Bb at a rate of around 40 arc seconds per year – not very fast but still about 30 times swifter than the highest proper motion star visible from Earth, Barnard’s Star. Combined with a huge parallax of about 130 arc seconds, it would certainly stand out among the other stars in the sky as viewed from 30 Ari Bb even if were not as bright as it is. If 30 Ari A possesses any planets, they would probably orbit within tens of arc minutes of the star and would require a telescope to spot.
With this little exercise completed, we get a taste of what the skies of planet would look like in a fairly “typical” quadruple star system. While the sun in the sky would be somewhat familiar looking, the noticeable factor of almost two variation in its apparent diameter and factor of three change in brightness over the course of a “year” would reflect the eccentric orbit of 30 Ari Bb. More alien to us would be the two incredibly bright stars present in the sky: the distinctly reddish 30 Ari C with a V magnitude of about -16 and the brilliant white 30 Ari A with a V magnitude of -12.3. The light from either of these stars is bright enough to cast shadows and they would be visible in daylight if they were in Earth’s sky. A telescope would reveal the tiny red companion of 30 Ari A as well as the disks of the stars and any orbiting planets. Naked eye observations could even reveal some planets orbiting the much closer 30 Ari C. Ignored in the simple calculations presented here (and which would add even more variety to the scene), is the fact that these parameters would vary on timescales of years to millennia due to the eccentric orbits the stars in this system surely possess. As fantastic as this view might appear to us, future discoveries are sure to find worlds with even more spectacular views in their skies.
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Related Reading
“Occurrence of Potentially Habitable Planets Around Red Dwarfs”, Drew Ex Machina, January 12, 2015 [Post]
“Architecture of M-Dwarf Planetary Systems”, Drew Ex Machina, October 24, 2014 [Post]
General References
E.W. Guenther et al., “A substellar component orbiting the F-star 30 Arietis B”, Astronomy and Astrophysics, Vol. 507, No. 3, pp.1659-1665, December 2009 [Paper]
Lewis C. Roberts, Jr. et al., “Know the Star, Know the Planet. III. Discovery of Late-Type Companions to Two Exoplanet Host Stars”, The Astronomical Journal, Vol. 149, No. 4, p. 118, March 2015 [Preprint]
“One Planet, Four Stars: The second known case of a planet in a quadruple star system”, Institute for Astronomy – University of Hawaii press release, March 4, 2015 [Press Release]
I find it interesting that the primary stars in this system appear to be in an oppositional gravitational orbit. I always assumed that multiple star systems would commonly have a large primary star, with smaller stars and planets orbiting the larger. I now wonder which type of system is the more common in binary star systems.
This is the best article I’ve read on Ari Bb. None of the others I’ve read ever think to mention the spectral class or luminosity of the star or the distance of the planet from the star. These are the things you need to know first to even make a wild guess as to the possibility of life as we know it on a world, or in this case, on a world-sized moon orbiting the planet.
I think about this a lot. You must have thought about it yourself.
In our system, Callisto is the only of the Galilean moons of Jupiter that orbits beyond reach of the ionizing radiation of Jupiter.
When I see that a world is five or six or even ten times the mass of Jupiter, I wonder how far out a large moon would need to orbit to escape its radiation field.
But that raises the question, “how does this make the temperature swing on the moon as it orbits the gas giant?”
Nobody anywhere else I’ve found has addressed these questions.
I went looking for this star at Wikipedia before I found your link. Even there the higher luminosity wasn’t mentioned.
But it did say:
“…this planet orbits only 0.005 AU (or 700,000 km) closer to the star than Earth to the Sun, but its orbital eccentricity is much higher compared to Earth. At periastron, the planet’s closest distance to the star is 0.708 AU, which is slightly closer to the star than Venus to the Sun. At apastron, the planet’s farthest distance to the star is 1.283 AU, which is more than half-way between the orbits of Earth and Mars.[2]”
So even if the star was the right temperature, this would make life really hard on a large moon of such a world. But thinking of all the extremophiles found on Earth in the hot springs of Yellowstone, Lake Mono CA, deep mines, or beneath the glaciers of Antarctica, could any life survive on a moon of 30 Ari Bb if 30 Ari B were a cooler star?