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Running Head: FORMATION AND ORBIT OF HOT JUPITERS
Formation and Orbit of Hot Jupiters
Amy A. Tracy
Salt Lake Community College
Phys 1040
Intro to Astronomy
M. Weiss, Professor of Astronomy
12/02/2016
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FORMATION AND ORBIT OF HOT JUPITERS
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Introduction
Hot Jupiters are planets that are larger than Jupiter, but are also located closer to their solar
system's star than Mercury is to our own sun. They can typically make an orbit around their star in just
a few days, and it is more commonly thought that they first formed further out in their solar system and
then migrated inwards towards their main star. Hot Jupiter's surface temperatures are usually around
6000 degrees Fahrenheit. Studying other solar systems and their hot jupiters helps us because we use
their migration pattern to figure out planets can move in other solar systems and try to find other
habitable planets (like our own Earth) and their orbits (Cain, 2015).
Formation – Migration Hypothesis
There are two main theories for the creation of Hot Jupiters. The more commonly accepted
theory is the Migration Hypothesis. In this hypothesis, when a star still has a solar nebula, Jupiter-sized
planets are formed. (Fifty percent of these planets had a companion planet nearby that had the same
mass or was larger.) Then the planet migrates up close to the star and establishes an orbit (Chambers,
2007). There are a few versions of the Migration Hypothesis too- one is that the Jupiter-planet
interacted with the nebular disk and was pulled inwards. Another idea is that sometime after the disk
went away, the planets were interacting with a third planet (or maybe a binary star) and the play
between the star and the two planets (or the two stars and the Jupiter) pulled the Jupiter close to the star
to establish an orbit (Cain, 2015).
Formation – In Situ
This less-favorable formation theory is that Hot Jupiters formed “in situ,” meaning in their
original place close to their star. It is thought that through core accretion theory, the cores of these
planets formed when the nearby dust and minute planets combined together to create Jupiter-sized
planets. There are some problems with this theory; usually there isn't enough debris closely around the
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star to be accumulated, and the debris should also be migrating inwards. But there are often super-Earth
type planets around the Hot Jupiter in this area, so they may be formed this way sometimes (Batygin et
al, 2015).
Retrograde
Hot Jupiter orbits are thought to start out really eccentric or elliptical and then they become
more circular over time as they stabilize around their main star. A lot of Hot Jupiters have retrograde
orbits, which causes astronomers to think of even more formation theories. We are not always sure if
it’s because their original orbit was interrupted by something, or if the star flipped due to magnetic field
interactions (Royal Astronomical Society, 2010). Like stated in Migration theory, it's possible that a
planet or a smaller star interacted with the Hot Jupiter, pulling it from it's usual orbit.
Misaligned Orbits
More than half of the Hot Jupiters that we’ve found have orbits that have a misaligned orbital
axis to the rotational axis of its star. The heat of the photosphere of the star might be what causes these
orbits to be off center. There are ideas that explain that Hot Jupiters that are higher temperature are
more likely to have misaligned orbital patterns. A team at Cornell University did several simulations of
Hot Jupiter orbits. The team believes that these orbit misalignments are most likely caused by an
interfering, smaller binary star (Glister, 2014).
Observed Orbits
Every Hot Jupiter orbit is different but we noticed that most of them last around 2-4 days. A
well recorded Hot Jupiter called 51 Pegasi b has an orbital period of about 4 days (Sahu et al, 2006).
Ultra-Short Orbital Period
Some Hot Jupiters are so close to their stars that they can orbit in just one day or less. The stars
they orbit are usually around 1.25 SM. We have discovered at least four Hot Jupiters that have
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extremely short orbits. They are WASP-18b, WASP-19b, WASP-43b and WASP-103b (Sahu et al,
2006).
Conclusion
Hot Jupiters tend to have other planetary bodies nearby, some of which are Earth-like. We use
Hot Jupiters to study orbital patterns of solar systems and to locate possible terrestrial planets that can
support life. We have discovered recently that the Hot Jupiter migration path may not be as destructive
to other planets as we previously thought. If the Hot Jupiter can stabilize it's orbit, then there are often
planets that move into the star's habitable zone. Our own Jupiter will probably become a Hot Jupiter in
the future when our Sun becomes a red giant. In 2018, The James Webb Space Telescope is launching
to study exoplanet atmospheres, which will include Hot Jupiters. We are still heavily researching these
planets in the search for life in the universe and to better understand our universe.
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References
Batygin, Konstantin; Bodenheimer, Peter H.; Laughlin, Gregory P. (2015, November 30th). In Situ
Formation and Dynamical Evolution of Hot Jupiter Systems. Cornell University Library.
arXiv:1511.09157. Retrieved from
https://arxiv.org/abs/1511.09157
Cain, Fraser. (2015, Dec 23). What are Hot Jupiters? UniverseToday. Retrieved from
http://www.universetoday.com/109269/what-are-hot-jupiters/
Chambers, John (2007, July 1). Planet Formation with Type I and Type II Migration. AAS/Division of
Dynamical Astronomy Meeting. 38. Bibcode: 2007DDA....38.0604C. Retrieved from
http://adsabs.harvard.edu/abs/2007DDA....38.0604C
Glister, Robert. (2014, September 16th). ‘Hot Jupiters': Explaining Spin-Orbit Misalignment. Centauridreams.org. Retrieved from
http://www.centauri-dreams.org/?p=31540
Royal Astronomical Society. (2010, April 13th). Turning planetary theory upside down. ESO Press
Release. Bibcode:2010eso..pres...16. Retrieved from
http://adsabs.harvard.edu/abs/2010eso..pres...16.
Sahu, K.C., Casertano, S., Bond, H.E., Valenti, J., Smith, T.E., Minniti, D., … Lubow, S. (2006).
Transiting extrasolar planetary candidates in the Galactic bulge [abstract]. Nature. National
Center for Biotechnology Information, U.S. National Library of Medicine. 443 (7111): 534–540.
FORMATION AND ORBIT OF HOT JUPITERS
arXiv:astro-ph/0610098. Bibcode:2006Natur.443..534S. doi:10.1038/nature05158. PMID
17024085. Retrieved from
https://www.ncbi.nlm.nih.gov/pubmed/17024085
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