Retrograde Exoplanets Challenge Theories

Perhaps one of the most fast-moving areas of modern astronomy research today is the study of extrasolar planets (or exoplanets). Extrasolar planets are planets orbiting stars other than our sun. There is great interest in the scientific community in attempting to find an earth-like planet orbiting a star outside our solar system. Though the research on exoplanets is often motivated by evolutionary concepts, the discoveries from this research are encouraging to a creation world view. Great advances have been made in recent years in the technology of the search for exoplanets. The count of the number of known exoplanets is now at over 500.1 All of these objects are within our own galaxy, the Milky Way. There have been certain characteristics found to be very common among the known exoplanets, such as the fact that many of them are gaseous planets similar to Jupiter and very close to their star. Compared to many of the exoplanets, Jupiter is relatively distant from our sun (over five Astronomical Units). There have also been interesting unusual cases. One of the most interesting recent findings is several exoplanets that show evidence of orbiting the star retrograde, that is, moving around the star in a direction opposite the spin of the star. These retrograde exoplanets pose a significant challenge to existing theories. The following examines some of the new observational methods applied today in studying extrasolar planets and relates recent discoveries to a young age creation viewpoint.

Retrograde exoplanets

Six retrograde extrasolar planets reported by the Royal Astronomical Society.
Photo credit: ESO/A. C. Cameron (www.eso.org)

Finding Exoplanets

In 2001 the author wrote a paper on extrasolar planets in which three methods of detection of these objects were described.2 Those methods were 1) the astrometric method, 2) the spectroscopic method, and 3) direct transit. Today all of these methods have been improved and often the data from multiple methods is combined to glean more details from the observations. There are also a few other techniques used today. It is important to treat the experimental evidence as a separate issue from the matter of the origin of stars and planets. Though Christians have reason to be skeptical regarding naturalistic origins theories, we can acknowledge experimental evidence and incorporate new discoveries in a creation perspective. Astrometric measurement (astrometry) simply attempts to use telescopes to precisely measure the star’s position. Over time, the wobble of the star due to the gravitational effect of the planets may be determined if the measurements are precise enough. Though earth-based telescopes have been used for astrometry for years for stars and dwarf stars, there has been very limited success using the technique for extrasolar planets. Extrasolar planets are usually significantly smaller than even dwarf stars and thus harder to observe directly. There are plans for space-based telescopes in the future that may make exoplanet astrometry more feasible. Astrometry would then have some advantages, such as for studying planets far from their stars.

The “spectroscopic method” is today usually called the radial velocity method (or RV method). It measures periodic variations in the doppler shift of the star light caused by planets causing the star to “wobble.”2 This measures variations in the speed of the star as it moves either toward or away from earth. This method is a well-established technique in astronomy. Even our own sun wobbles like this due to the gravitational pull of the planets in our solar system.2 The radial velocity method is not well suited to detecting small planets, since such planets may not cause a significant wobble in the star's motion. Planets that are farther from the star require more observations over a longer period of time. Today, more exoplanets are still detected by the RV technique than by any other method. According to Wikipedia, 70 out of 85 exoplanets detected in 2009 were discovered by this method.3

The second-most-used technique for detecting extrasolar planets is the transit method. This method measures a dip in the light of the star as the planet passes in front of it, from our line of sight at earth. The transit technique can only be used for fortunate cases where the planet orbit crosses our line of sight to the star. However there is great interest in this method because it allows scientists to determine the size of the planet and to get some information about the planet's atmosphere. Often the radial velocity and transit methods are both used to study particular objects. The transit technique confirms that the object is definitely a planet and also gives its mass and diameter. Currently there are over 110 extrasolar planets that have been studied by transit measurements.4 An important new space-based telescope, known as Kepler, is designed to do transit measurements of exoplanets at an unprecedented level of precision. Kepler was launched in 2009 and is just outside earth's orbit, trailing behind earth. It is believed that Kepler will be able to do transit measurements of earth-sized planets, if any are found. Some scientists argue it may even be possible for the Kepler telescope to detect moons orbiting extrasolar planets.

To date, the closest exoplanet to being earth-sized is one called Gliese-581e, which is about twice earth’s mass.5 Note that, for smaller objects such as Gliese-581e, the uncertainties in the method may lead some astronomers to question whether there has been a reliable detection of a planet. Gliese-581 is a system with four objects generally accepted as being planets. Gliese-581e is too hot to be habitable. Currently earth is still the only truly “earth-like” planet known. In 2010, one group of researchers announced discovery of two additional planets in the Gliese 581 system, labeled as objects f and g. One of them, 581g, was initially called the first “Goldilocks planet,” the first exoplanet believed to be in the habitable zone for that star.6 Later efforts, however, by another team of researchers were not able to confirm the detection of objects f or g.7 Thus it is important to not jump to hasty conclusions after initial reports.

A recent development in extrasolar planet research is what is called direct imaging. This is where very sensitive space-based instruments are able to actually get an image of an exoplanet. This is not just measuring the star's position and inferring the planet indirectly, but actually getting a picture of the exoplanet. One of the first direct imaging observations was done using the Spitzer Space Telescope in 2004.8 This was observing infrared light from a star with a planet. In 2008, the Hubble Space Telescope used its Advanced Camera for Surveys to directly observe the motion of a planet around a bright nearby star known as Fomalhaut9 This device on Hubble detects ultraviolet and visible light. It also uses a coronagraph, which blocks out the bright light of the star so the planet can be seen. There is great interest in Fomalhaut and its planet because this system is relatively nearby and possesses a dust ring as well as the planet. Fomalhaut is brighter than our sun and the dust ring begins at about 137 A.U. from the star (1 Astronomical Unit is approximately 93 million miles, the sun-earth distance). The planet, referred to as Fomalhaut b, lies at about 119 A.U. from Fomalhaut.10

Photo credit: NASA, ESA, and P. Kalas (University of California, Berkeley) (hubblesite.org)

Fomalhaut brings up the question of whether planets form from dust disks. Scientists tend to assume so, but this has not actually been observed. The fact that the planet and the dust ring both exist in the same system does not necessarily mean the dust had something to do with the formation of the planet. The Fomalhaut b planet is in a somewhat elliptical orbit and the dust ring is also somewhat elliptical, probably because of the planet sweeping up dust. However, there are still unanswered questions about this system and the orbit of the planet is not known with much precision. The long axis of the planet orbit and of the dust ring are apparently not aligned as expected. There is a need to better define the planet orbit as well as what is happening in the dust ring. Further research on the Fomalhaut system may provide more insight in coming years.

Other exoplanet observation techniques have been refined and developed that help scientists find more exoplanets and get better data. One example is Nulling Interferometry, 11 which allows scientists to use multiple telescopes so that they act like one larger telescope and also provides a way to cancel out the bright light from the star. This technique is beginning to provide images of star systems with planets. Another technique applies principles from Einstein's General Theory of Relativity. It is known as Gravitational Microlensing. It involves a distant star and a “foreground” star not as distant. If the alignment with our line of sight is just right, the light from the distant star gets magnified by the foreground star due to the gravity of the foreground star. When this happens, if a planet orbiting the foreground star comes into the right position it will make a variation in the light from the distant star. This technique has only been used to detect a few planets. The Microlensing technique can be well suited for detecting planets far from their stars. Also the Microlensing technique could be applicable to star systems much farther away from our sun. Scientists continue to extend the limits of exoplanets they can detect.

Origins and Exoplanets

The naturalistic evolutionary model for the formation of our solar system holds that a disk of gas and dust once surrounded our sun and the planets and many smaller objects in our system formed from material in this disk. Thus this “accretion disk” as it is called becomes the common source for all the objects in our solar system. It is generally believed that the temperature in the disk determined why planets near the sun are high density rocky planets and the more gaseous planets with lower densities are found farther from the sun. However when scientists began discovering extrasolar planets, this theory did not work well because many extrasolar planets are too close to the star for the same process to work. Many exoplanets are now referred to as “Hot Jupiters” because they are large gaseous planets similar to Jupiter but their orbits put them very near the star. Being near the star, the temperature for many of these Hot Jupiters is too great for gases such as hydrogen or helium to condense onto the planet. Thus, scientists began exploring other origins scenarios that would allow a Jupiter-like planet to form several astronomical units distance from the star and then move inward. This idea, now generally accepted, is known as orbit migration.12

In orbit migration, while planets are pulling together from a disk of dust and gas, part of the disk is massive enough to cause the newly forming planet's orbit to change. Thus the disk can theoretically cause the planet to migrate or move either inward or outward from the star. When the disk dissipates and becomes too thin, it would stop modifying the planet orbit. Inward migration would be assumed to be more common. Planets move in the same direction in their orbits as the disk spins because the planets get their orbital energy from the disk. Planets by these ideas would normally form in circular orbits moving in their orbit in the same direction that the star spins. The planet and the star are believed to come from the same disk, thus as the star and the planets form the spin of the star would be in the same direction as the planet revolves in its orbit. When multiple planets are in a system, their mutual gravitational pulls on each other can modify their orbits so they may not always stay in simple circular orbits. One theory proposed for our solar system suggests that Neptune and Uranus formed nearer to the sun than their present locations and then they migrated outward farther from the sun because of the motions of Jupiter and Saturn.12 By this theory, in time our system of planets came into stable orbits like they are in today.

There are a wide variety of types of star systems now in which extrasolar planets have been found. In a few systems the planets orbit pulsars or brown dwarf stars instead of normal stars.13^,14 Some systems with planets are binary or trinary star systems.15^,16 This means there are 2 or 3 stars orbiting each other and the planet orbits one or more of these stars. Some of the exoplanets are in unstable orbits because they are so close to the star. One example was discovered in 2008 and further studied by the Hubble Space Telescope in May 2010. This planet is called WASP-12b.17 WASP is an acronym for Wide Area Search for Planets, which uses two special telescopes to do an automated search for transiting planets. WASP-12 is a yellow dwarf star approximately 600 light-years from earth.17 The planet, WASP-12b, is so near the star that matter is being pulled off the planet, creating a significant cloud of material around the planet that falls into the star. The planet is probably stretched out in shape similar to a football. It has been estimated WASP-12b could be essentially eaten up by its star in only 10 million years. The planet and the star could thus be only thousands of years old. There are other similar examples of exoplanets that cannot last long where they are found near their stars.18

One planet has been found that seems to be a rocky planet similar to earth in density.19 This planet is known as CoRoT-7b. Much research has been done on this planet; both its mass and its size have been determined, making its density determined to be 5.6 ± 1.3 g/cm³.20 This makes it very close to earth's density and thus likely rocky in nature. There has been various estimates of its mass but it is generally believed to be about 5 times the mass of earth. It is about 490 LY from earth orbiting a star slightly smaller than our sun. It may be closer to being like earth than any other known exoplanet; however, it is extremely hot because it orbits its star in only about 20 hours. It is much closer to its star than Mercury is in our solar system. So, it would not be suitable for any kind of life. Scientists search for a planet in what is known as the “habitable zone.” To define the “habitable zone,” scientists look for a planet where the temperature based on its distance from the star would allow for liquid water. They also look for carbon dioxide in the atmosphere. However, these two characteristics are not all that is necessary for a planet to be habitable. Even if a planet had liquid water and carbon dioxide there could be other reasons the planet would not be habitable. So far scientists have not found any world that would be considered habitable, though there has been debate about certain exoplanets possibly being in the “habitable zone.”

Exoplanet orbits are also often unlike the planet orbits in our own solar system. In our solar system, all the planet orbits are all fairly close to being circular. A perfectly circular orbit would have an eccentricity of 0, as astronomers measure orbits. On the other hand the more elongated or stretched out a planet orbit is, the closer the eccentricity gets to 1. The most eccentric orbit of the 8 planets in our solar system is that of Mercury, at 0.2. Among extrasolar planets, though many are circular, some of their orbits to have eccentricities of 0.5 or more.3 In our solar system all the planet orbits are inclined such that they are near the plane of earth's orbit (this plane is called the ecliptic). This, along with the near circular shape of the orbits tends to keep the planets in stable orbits in our system. The planets in our system are not likely to pull each other into other unstable motions. In extrasolar systems, many planet orbits are inclined at significant angles compared to the tilt of the star, even over 70 or 80 degree angles in many cases.3

On April 12, 2010 the Royal Astronomical Society published a press release called “Turning Planetary Theory Upside Down.”21 In the press release it reports six cases of extrasolar planets that were found to be orbiting their stars in a direction opposite the direction of spin of their stars. It had been already noted sometimes that some exoplanets orbits were tilted at significant angles compared to their stars, and this is problematic enough for planet origins models. But to make an even greater challenge, these six exoplanets had their orbits inclined more than 90 degrees in angle compared to the equators of their stars. The retrograde orbital motion is detected by examining the spectra from the stars for something called the Rossiter-McLauphlin Effect.22 The way the planet crosses the edges of the star as it passes in front of the star shows a kind of imbalance in the redshift curve if the planet is moving retrograde. A number of researchers from different observatories got involved in doing the analysis of these planets. One of these scientists, Andrew Cameron from the University of St. Andrews in Scotland, said, “The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their star's spin.”21 Much research effort has gone into developing theory for how a dust disk around a star could cause forming planet orbits to migrate. But there is no way a dust disk, which must be aligned with the star's equator, can cause planet orbits to tilt by over 90 degrees. Of course, from a creation perspective, God can create planets orbiting their star at any inclination He pleases! But how will scientists who are insisting on a naturalistic explanation explain the retrograde planets?

Planet Scattering

Scientists will be working on explaining the retrograde exoplanet orbits for some time. But there is one known dynamical effect that astronomers are beginning to consider as a possible explanation. It is from an effect called Kozai cycles.23 The theory of the Kozai mechanism comes from the study of multiple star systems. In a star system where there are two stars near each other and a third more distant star orbiting the other two, the distant star can cause the orbits of the inner stars to oscillate. The inclination of the orbit of the inner stars as well as the elliptical shape of the orbits can vary periodically in response to pulls from the orbit from an outer star with a very inclined orbit. Thus the orbit of an inner object or objects can be caused to oscillate because of a more distant object. It is thought that extrasolar planets could do the same thing in some systems. It is a rather complicated process.

If a system has only one planet and one star, the Kozai cycles mechanism would not be applicable. The whole scenario would involve a dust disk, three or more planets or stars interacting by their gravitational pulls, and tidal effects due to a star on a nearby planet. Scientists might argue that in the retrograde exoplanet systems, in the past there could have been another distant planet that altered the orbit tilt of an inner planet until other effects stopped the Kozai cycles. Tidal forces between a star and a planet close to the star can be very significant and might theoretically round off an elliptical orbit and stop the Kozai orbit oscillations. Thus, scientists are now looking to see if any of the known cases of retrograde exoplanets are in systems that have multiple stars or have other more distant planets that could make the Kozai mechanism or similar processes applicable. These type of processes are referred to as planet-planet or star-planet scattering. Such a process would likely take hundreds of millions or billions of years for the multiple stars and planets in a complicated system to slowly change into the system we see.

A question to ask regarding the six retrograde exoplanets is, will complicated planet-planet-star interactions like the Kozai mechanism work? The six exoplanets reported by the Royal Astronomical Society are all from the WASP research program. They are called WASP-2b, 5b, 8b, 15b, 17b, and 33b. Note that another retrograde planet was also reported earlier by other researchers from Japan in 2009, that planet is called HAT-P-7b.22 Of the six WASP exoplanets reported by the Royal Astronomical Society, WASP-8b is a trinary (3 star) system and WASP-2b is believed to be a binary star system (2 stars). One source suggests that WASP-33 may have a “third body” but apparently little is known about the third object. So, of the six retrograde exoplanets above, for at least three of these there is no evidence that any planet-planet or star-planet scattering would apply to these systems. The HAT-P-7 system also is a simple system, as far as is now known, with one planet orbiting one star. Scientists tend to assume some kind of planet-planet scattering process was at work in these systems even if there is currently no evidence of additional stars or planets that would cause such scattering.

Such scattering theories suggest that a planet near the star had its orbit tilted because of a more distant planet or star that was also in a very inclined orbit. But in this scenario, what caused the tilt of the outer object's orbit? The inclination of the distant outer object is also a challenge to naturalistic origins theories. To always suppose that there was another planet or star at a distance that caused an orbit tilt of an inner object will obviously not always work. There will not always be a distant companion star or planet in the proper place to be the “cause” of inclined planet orbits.

The research on the retrograde exoplanets will continue and scientists will undoubtedly discover more interesting things about extrasolar planets. It is important for us to remember God created all these stars and planets. They show God's glory and demonstrate how special our own home planet is. Creationists can appreciate that the God of the Bible made everything in the universe to reflect his glory and creativity. He is not limited to the familiar things we know of on earth or to the characteristics of the planets in our solar system. Outside our solar system, it may be that God created not to provide for life but just to show his greatness to mankind on earth. Our planet and even our solar system are created with purpose to give us a safe stable environment. This should motivate us to want to know and worship our Creator.