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Journal of Creation 35(2):11–13, August 2021

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Confirmations of highly inclined exoplanet orbits

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planet-2

There has been increasing interest and new research into extrasolar planets that have highly inclined orbits compared to the orientation of their stars. This was first referred to by Spencer in 20101 for the case of an exoplanet known as WASP-17b (The acronym ‘WASP’ represents a British project called the ‘Wide Area Search for Planets’). In theories of the formation of planets, the star first forms in a flattened disk of gas and dust. The planets then form from the gas and dust in the spinning disk. Since the newly formed planets in this scenario would get their motion from the spinning disk, the planet orbits would normally be expected to be lined up close to the equator of the star. But in some cases exoplanets have been discovered in which the orbit inclinations are very different from the plane of the equator of the star. This raises questions about how these exoplanetary systems came into this configuration. These cases have been challenging for planetary scientists to explain. In our own solar system, this angle between the solar equator and the ecliptic plane of the planets is approximately 7°.

In mid-April of 2010, a conference was held at the University of Glasgow for astronomers. This event was the National Astronomy Meeting of the Royal Astronomical Society (RAS). At this meeting it was announced that scientists had discovered that in a group of 27 exoplanet cases studied, six of these planets seemed to be moving retrograde in comparison to the spin axis of their star. This was quite a surprise.2 At the conference, the report on the six retrograde exoplanets was presented by Andrew Cameron. He made the statement: “The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin.”2

Measuring misalignments

Since 2010 there has been more research on similar cases, and scientists are attempting to refine their methods for investigating these stars and their exoplanet orbits. Today these exoplanets are referred to by scientists as ‘misaligned’ when the plane of the exoplanet’s orbit is significantly different from the equator of the star. There are many factors that complicate observations of the inclined exoplanets. Multiple types of data have to be combined in order to properly determine the properties of the star. Once the star’s spin axis can be determined with some confidence, then it may be possible to determine the angle between the stellar spin axis and the exoplanet orbit. Some key techniques involved in these determinations are asteroseismology, spectroscopy measurements of the transit light curve, and what is known as the Rossiter-McLaughlin effect (figure 1).

Image: Autiwa / CC BY 2.5fig-1-rossiter-mclaughlin-effect
Figure 1. In the graphic an Earth observer looks at the rotating star from the bottom. The star will have blueshifted light on the side of the yellow disk that moves toward the observer and redshifted light on the opposite side. When an exoplanet passes in front of the star it will first block the blueshifted light (middle picture) and then block the redshifted light (right picture). The change this causes in the redshift is known as the Rossiter-McLaughlin effect.

The first requirement in determining the inclination of the exoplanet orbit is that the planet must transit the star along our observational line of sight. (In a transit, the exoplanet passes in front of the star.) The transiting planet causes a dip in the light curve from the star. Asteroseismology is then used to analyze oscillatory modes of the star to determine a number of stellar properties, including its spin axis.3

The Rossiter-McLaughlin effect affects Doppler measurements of redshift during a transit. Due to the star’s rotation, one side of the stellar disk moves toward the observer (and is blueshifted) while the other side of the star moves away from the observer (redshifted). As the planet passes in front of the stellar disk, it blocks part of the blueshifted light and then blocks part of the redshifted light (figure 1). This produces a measurable effect in the redshift at the edges of the star.4,5 This causes the redshift to be skewed in a manner that depends on the angle between the orbit of the planet and the spin axis of the star. The actual geometry is complicated because in doing an observation, the observer is seeing a projection of the stellar spin axis and a projection of the exoplanet’s orbital plane. Thus, the variations in the star and the Doppler transit data are examined over a period of time and a model of the motion of the star and the planet is constructed to explain the variations in the light curve.

There are two angles used in reference to these cases, one is denoted by the Greek letter λ (lambda), which is the projection of the spin-orbit angle on the plane of the sky. This angle is what is observed but the actual angle desired is in a different plane. So, the goal is to determine in three dimensions the angle between the stellar spin axis and the planet’s orbital axis, denoted by ψ (psi).3 The angle ψ is known as the stellar obliquity. Only some transiting exoplanets have been sufficiently studied to determine both of these angles. The results can vary from different research teams measuring the two angles. In 2017 Spencer listed 20 cases of inclined exoplanet orbits for 16 stars.6 In those cases, the ψ angle ranged from 0 to 145°. (Note that an angle ψ greater than 90° is considered retrograde, although exoplanet cases close to 90° are often described as being in ‘polar orbits’.)

It is instructive to consider how much variation in the λ and ψ angles there can be from different observations and different analyses. A concern might be raised over whether the conclusions correctly follow from the observations. Can the high angles for λ and ψ be reproduced? To show an example we can consider exoplanet HAT-P-7b. Exoplanet HAT-P-7b may be one of the most studied cases of exoplanets and thus multiple teams have done observations and analyses. Table 1 shows four sets of values for the λ and ψ angles, along with the amount of possible error or uncertainty estimated by the various researchers. In table 1, each name under the ‘Source’ column represents a team of researchers with a published source. The Winn7 and Narita8 sources, as well as Albrecht5 all did their own original observations of the HAT-P-7 star. Later, the Lund3, Benomar,9 and Campante10 teams re-analyzed the data. Note that the uncertainties in both angles is significant and is estimated very differently by different researchers. Yet, in spite of this, the final results for angle psi are similar, with the angle estimated to be from 94.6° to 116.4°. Thus, this planet is in a roughly polar orbit around its star. This conclusion is certainly borne out by multiple researchers.

Table 1. Results from RM measurements for the HAT-P-7b transit. Bracketed references are for earlier teams who did original observations which were subsequently reanalyzed by others. Psi is the estimated actual angle between the angular momentum vectors of the stellar spin axis and the planet orbit.

table-1-RM-measurements-hat-p-7b-transit

Origins of inclined exoplanet orbits

In the accepted origins scenarios for the formation of stars, disks, and planetary systems, this raises interesting questions. From a creation perspective, a Creator could create an exoplanet with any orbit inclination about its star. But from a naturalistic or evolutionary perspective, it requires scientists to put forward creative scenarios. One common approach is to propose that a planet near the star, as it migrated in, would have its orbit altered over long periods of time by another planet (or possibly a second star) in a more distant but inclined orbit. This is sometimes referred to as the Kozai effect.11,12 The Kozai mechanism relies on there being a few planets or objects in orbits that are in differing planes that can perturb each other over time. A planet migrating inward can be influenced by tidal effects from the star as well. Near passes between planets can also cause an orbit inclination to change. Planet–planet scattering scenarios like this tend to require billions of years of time. Planet–planet scattering scenarios also require that there be a third object (a planet or star) in certain types of inclined elliptical orbits. In many systems there is no observational evidence of such a third object.

Other scenarios have been put forward that try to provide a mechanism for the star and the disk around it to be misaligned early during the formation of the star. One approach to this suggests the magnetic field of the star can tilt the star relative to the disk.13 Another proposal is that in some systems if gas and dust falling onto a forming star falls inward with a non-symmetrical configuration it can cause the resulting star to have a different orientation than the resulting disk.13 Another view suggests that the star formed in a cluster of multiple newly forming stars and the interaction of the stars near each other distorts, truncates, and tilts the disk before the planets form.14 Then the planet (or planets) form from what remains of the disk. These types of scenarios might be described as ‘early tilt’ mechanisms. In these ‘early tilt’ scenarios there is less reliance on exoplanet orbit migration. Instead, the orientation of the star compared to the disk is determined early while the star is young and then the planets form later. A potential problem with these scenarios is that the disks tend to be disrupted by the processes and so there may not be enough material left to form planets. Also, all of these early-tilt scenarios depend on complex theories of star formation which are totally theoretical and not verifiable by observations.

A number of researchers have noticed another curious correlation between the effective temperature of the star and its obliquity angle ψ. It seems that hotter stars tend to have more highly tilted orbits for their exoplanets. More research is needed on potential causes of this correlation. Some researchers argue it relates to the ‘early tilt’ star formation scenarios above.15 However, Winn16 and Albrecht5 have suggested it could be a tidal dissipation effect relating to the interaction of the star and the planet. As the planet migrates inward toward the star tidal forces become stronger. It is thought that a hot star is more likely to change the tilt of a planet orbit but a cooler star may come into alignment with the planet orbit.

Conclusions

The exoplanets with misaligned orbits will continue to motivate much research by planetary scientists. The critical examination of theories it motivates is a healthy exercise in science. How many exoplanets have these ‘misaligned’ orbits? There are various estimates, but it may be that approximately 40% of transiting exoplanets which have been studied using the RM measurements could be considered misaligned. Out of this 40%, only a few are likely to be actual retrograde cases. Benomar et al.9 summarize the problem:

“Among the 70 transiting planetary systems observed with the RM effect, more than 30 systems exhibit significant misalignment with |λ| > 22.5° … . This unexpected diversity of the spin–orbit angle is not yet properly understood by the existing theories, and remains an interesting challenge.”

That there are highly inclined exoplanet orbits (relative to their stellar equators) is not disputed today among planetary scientists. This finding has challenged exoplanet formation theories and has led to new theories to attempt to explain these cases. The diversity of exoplanets points to God’s creativity and power. Though Scripture does not mention planets in Genesis Chapter 1, it seems logical to put the supernatural creation of planets along with the creation of stars on the fourth day of the Creation Week. Without acknowledging supernatural creation, scientists are forced to propose very complex scenarios to explain exoplanet diversity. The variety of planetary systems God created continues to surprise and challenge scientists. Supernatural creation as outlined in Genesis is still a viable explanation.

Posted on homepage: 28 October 2022

References and notes

  1. Spencer, W., The search for Earth-like planets, J. Creation 24(1):72–76, 2010. Return to text.
  2. Turning planetary theory upside down, eso1016 Science Release of 13 April 2010, eso.org/public/news/eso1016/, accessed 30 April 2021. Return to text.
  3. Lund, M.N. et al., Asteroseismic inference on the spin-orbit misalignment and stellar parameters of HAT-P-7, Astronomy & Astrophysics 570, A54, 2014. Return to text.
  4. Bayliss, Daniel D.R. et al., Confirmation of a retrograde orbit for exoplanet WASP-17b, Astrophysical J. 722.2:L224–L227, 2010 ǀ doi:10.1088/2041-8205/722/2/l224. Return to text.
  5. Albrecht, S., et. al., Obliquities of hot Jupiter host stars: evidence for tidal interactions and primordial misalignments, Astrophysical J. 757(1), A18, 2012 ǀ doi:10.1088/0004-637X/757/1/18. Return to text.
  6. Spencer, W., The challenges of extrasolar planets, CRSQ 53:272–285, 2017. Return to text.
  7. Winn, J.N., et. al., HAT-P-7: A retrograde or polar orbit, and a third body, The Astrophysical J. 703:L99–L103, 2009. Return to text.
  8. Narita, N. et al., First evidence of a retrograde orbit of a transiting exoplanet HAT-P-7b, Publications of the Astronomical Society of Japan 61:L35–L40, 2009. Return to text.
  9. Benomar, O., Masuda, K., Shibahashi, H., and Suto, Y., Determination of the three-dimensional spin-orbit angle with joint analysis of asteroseismology, transit light curve, and the Rossiter-McLaughlin effect: cases of HAT-P-7 and Kepler-25, Publications of the Astronomical Society of Japan 66(5), A94, 2014 ǀ doi:10.1093/pasj/psu069. Return to text.
  10. Campante, T.L. et al., Spin-orbit alignment of exoplanet systems: ensemble analysis using asteroseismology, The Astrophysical J. 819(1), A85, 2016 ǀ doi:10.3847/0004-637X/819/1/85. Return to text.
  11. Fabycky, D. and Tremaine, S., Shrinking binary and planetary orbits by Kozai cycles with tidal friction, The Astrophysical J. 669:1298–1315, 2007; arxiv.org/abs/0705.4285. Return to text.
  12. Plavchan, P. and Bilinski, C., Stars do not eat their young migrating planets: empirical constraints on planet migration halting mechanisms, The Astrophysical J. 769, A86, 2013 ǀ doi:10.1088/0004-637X/769/2/86. Return to text.
  13. Lai, D., Foucart, F., and Lin, D.N.C., Evolution of spin direction of accreting magnetic protostars and spin-orbit misalignment in exoplanetary systems, Monthly Notices of the Royal Astronomical Society 412:2790–2798, 2011 ǀ doi:10.1111/j.1365-2966.2010.18127.x. Return to text.
  14. Bate, M.R., Lodato, G., and Pringle, J.E., Chaotic star formation and the alignment of stellar rotation with disc and planetary orbital axes, Monthly Notices of the Royal Astronomical Society 401:1505–1513, 2010 ǀ doi:10.1111/j.1365-2966.2009.15773.x. Return to text.
  15. Louden, E.M. et al., Hot stars with Kepler planets have high obliquities, The Astronomical J. 161, A68, 2021, arxiv.org/abs/2012.00776. Return to text.
  16. Winn, J.N., Fabrycky, D., Albrecht, S., and Johnson, J.A., Hot stars with hot Jupiters have high obliquities, The Astrophysical J 718.2: L145–L149, 2010 ǀ doi:10.1088/2041-8205/718/2/l145. Return to text.

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