Planetary system orientation & the search for exoplanets

Planetary system orientation & the search for exoplanets

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A quick review of photographs of galaxies will reveal that not all galaxies are orientated in the same plane as the field of view from Earth. Some galaxies are parallel to our field of view and we see them side on. Others are perpendicular to our field of view and we see them as if we are looking either from above or below them.

Given this fact, it would be fair to assume that the orientation of planetary systems is similarly varied. Not all of them are parallel to our field of view.

All the exo-planetary systems I am aware of have been seen in a side view perspective and any exoplanets have been detected by either a wobble of the star at the center of the planetary system or via dimming of the star's light as the exoplanet transits the star.

Have any exo-planetary systems been discovered that are perpendicular to our field view, looking from above or below the system and if so, how have any exoplanets in these systems been detected? If no such systems have been observed, how would astronomers detect exoplanets in such systems?

The multi-planet system HR 8799 is pretty close to face-on. Here's a video made by interpolating images taken over 7 years showing the orbital motion.

The planets of HR 8799 were detected by imaging. The majority of exoplanets have (at the time of writing) been detected by the radial velocity method, which requires radial motion and thus is most sensitive to planets close to edge-on, and the transit method which requires that the planet passes in front of the star as seen from Earth (thus will only spot planets extremely close to edge-on). The imaging method does not have this issue, so can find both face-on planets (like HR 8799) and edge-on ones, like Beta Pictoris b (video).

Astrometry is another exoplanet detection method that would allow detection of face-on systems. It has been plagued by a number of false starts (e.g. the infamous "detection" of planets around Barnard's Star by Peter van de Kamp) but hopefully once the full Gaia dataset is released there will be numerous astrometric exoplanet detections, which may include near-face-on systems.

For an overview of the many methods: Wikipedia - gravitational microlensing.

Each detection method has a different sensitivity to the inclination of the exo-planet's orbit:

  • Radial velocity is most sensitive to side-on planetary orbits as it looks for changes in velocity along the line of sight. Purely face-on planets are undetectable. You need the inclination to get the mass of the planet rather than lower limit on mass.
  • Transit can only detect planets nearly side-on.
  • Imaging does not depend much on inclination, but the system must be big and close.
  • Astrometry does not depend much on orbital inclination as it measures side-to-side motion of star against star.

  • The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such.
  • The first exoplanet detected was Gamma Cephei Ab in 1998. It was not confirmed until 2003 though, and generally, it is believed that the two planets orbiting the pulsar PSR 1257+12 are the first confirmed discovery. The discovery was announced in 1992.
  • The first planet discovered orbiting around a Sun-like star was 51 Pegasi b.
  • In respects to a planet similar to Earth, the first was discovered in 2014 named Kepler 186f. It is also located in the habitable zone of its star.
  • The habitable zone is a region around a star where liquid water could exist on the surface of a solid planet.
  • All the known exoplanets fall into categories depending upon their size, mass, and orbital positions.
  • The closest star system and closest planetary system to the solar system is at 4.37 light-years away. It is named the Alpha Centauri system.
  • If life is discovered on other planets, then the panspermia hypothesis would greatly become more likely the answer on how life on Earth was established.
  • There are over 4.000 exoplanets that have already been confirmed.
  • Almost 5.000 other exoplanets are candidates waiting to be approved.
  • Over 3.000 planetary systems have been discovered.
  • About 700 systems have more than one planet.
  • There are about 5 methods used for the detection of exoplanets. A sixth one has been implemented recently.
  • In 2009 NASA launched a spacecraft named Kepler, specifically to look for and detect exoplanets.
  • Among exoplanets, rogue planets have also been detected. They are planets that have been pushed out of their star system.

Since our knowledge of the Universe has expanded, searching for planets outside our Solar System has been a priority in the search for extraterrestrial planets and new habitable worlds that might sustain us.

The question of whether or not we are alone in the universe, or if there are other places such as Earth, may have well begun since our world has been mapped, and mostly explored. The first possible evidence of an exoplanet was noted in 1917, but it was quickly dismissed.

As technology progressed, many other possible discoveries of exoplanets were proposed as early as 1988 but were only later confirmed. In 1992 the first detection of an exoplanet was finally confirmed. The radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.

Ever since then, the discovery of exoplanets became somewhat of a priority, and naturally, a great interest to humanity. Technology progressed and new methods of detection were implemented specifically for this task.


WASP consists of two robotic observatories SuperWASP-North at Roque de los Muchachos Observatory on the island of La Palma in the Canaries and WASP-South at the South African Astronomical Observatory, South Africa. Each observatory consists of an array of eight Canon 200 mm f1.8 lenses backed by high quality 2048 x 2048 science grade CCDs, the model used is the iKon-L [6] manufactured by Andor Technology. [7] The telescopes are mounted on an equatorial telescope mount built by Optical Mechanics, Inc. [8] The large field of view of the Canon lenses gives each observatory a massive sky coverage of 490 square degrees per pointing. [9]

The observatories continuously monitor the sky, taking a set of images approximately once per minute, gathering up to 100 gigabytes of data per night. By using the transit method, data collected from WASP can be used to measure the brightness of each star in each image, and small dips in brightness caused by large planets passing in front of their parent stars can be searched for.

One of the main purpose of WASP was to revolutionize the understanding of planet formation, paving the way for future space missions searching for 'Earth'-like worlds.

WASP is operated by a consortium of academic institutions which include:

On 26 September 2006, the team reported the discovery of two extrasolar planets: WASP-1b (orbiting at 0.038 AU (6 million km) from star once every 2.5 days) and WASP-2b (orbiting three-quarters that radius once every 2 days). [11]

On 31 October 2007, the team reported the discovery of three extrasolar planets: WASP-3b, WASP-4b and WASP-5b. All three planets are similar to Jovian mass and are so close to their respective stars that their orbital periods are all less than two days. These are among the shortest orbital periods discovered. The surface temperatures of the planets should be more than 2000 degrees Celsius, owing to their short distances from their respective stars. The WASP-4b and WASP-5b are the first planets discovered by the cameras and researchers in South Africa. WASP-3b is the third planet discovered by the equivalent in La Palma.

In August 2009, the discovery of WASP-17b was announced, believed to be the first planet ever discovered to orbit in the opposite direction to the spin of its star, WASP-17.

The discovery of the J1407 system and its unusual eclipses were first reported by a team led by University of Rochester astronomer Eric Mamajek in 2012. [14] The existence and parameters of the ring system around the substellar companion J1407b were deduced from the observation of a very long and complex eclipse of the previously anonymous star J1407 during a 56-day period during April and May 2007. [14] [15] The low-mass companion J1407b has been referred to as a "Saturn on steroids" [16] [17] or “Super Saturn” [18] due to its massive system of circumplanetary rings with a radius of approximately 90 million km (0.6 AU). [19] The orbital period of the ringed companion J1407b is estimated to be around a decade (constrained to 3.5 to 13.8 years), and its most probable mass is approximately 13 to 26 Jupiter masses, but with considerable uncertainty. [19] The ringed body can be ruled out as being a star with mass of over 80 Jupiter masses at greater than 99% confidence. [19] The ring system has an estimated mass similar to that of the Earth. [20] A gap in the ring system at about 61 million km (0.4 AU) from its centre is considered to be indirect evidence of the existence of an exomoon with mass up to 0.8 Earth masses. [19]

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The Exoplanets and Stellar Astrophysics Laboratory studies the formation and evolution of stars and planetary systems using advanced telescopes and theoretical techniques. We develop new technology and ideas that help us understand our place in the universe. Research also contributes to the search for Earth-like planets and habitable environments around other stars. Key areas of research include exoplanet searches and theoretical modeling, stellar debris disks, young stellar objects, stellar astrophysics, and numerical modeling of planetary system evolution.

Astrophysical Research:
Extrasolar Planets, Debris Disks, Young Stellar Objects, Stellar Astrophysics, Theory, Coronagraphic imagers, Ultraviolet detectors, Integral Field Spectroscopy

Hubble, Herschel, LBTI, TESS, JWST, WFC3, STIS

Future Mission Concepts

Our Exoplanets Seminar Series meets Tuesdays at 11:30 in Build. 34, Rm. E215.

The Stellar and Extragalactic Astronomy Lunch meets Thursdays at noon in Build. 34, Rm. E215.

Exoplanet Atmospheres

In 2005, Spitzer became the first telescope to allow astronomers to directly detect the light from hot Jupiters. Light curves from two exoplanets, TrES-1 and HD 209458b, clearly showed the signs of the planetary eclipses as they passed behind their stars. This revealed the amount of light coming from the exoplanets, which revealed vital clues to their temperatures and atmospheric properties.

A spectrograph lets astronomers split up light into its various wavelengths, much like the way light through a prism lets us see each color individually. The resulting spectrum can reveal the unique fingerprints of different chemicals.

Astronomers can study more than just the overall brightness of an exoplanet when it is eclipsed by its star they can also obtain its spectrum. A spectrum is created when we split light into its different colors, in much the same way that a prism splits light into the colors of the rainbow, but down to much finer wavelengths. To capture an exoplanet's spectrum, an infrared telescope has to observe the planetary system twice using a spectrograph. The first time it takes a spectrum of the host star together with the exoplanet. Then it takes a spectrum of just the host star, when the exoplanet has disappeared behind it. By subtracting the spectrum of the host star from the combined spectrum of the star plus the exoplanet, astronomers are able to get the spectrum for just the exoplanet.

The spectra can let us identify the unique fingerprints of materials like hydrogen, carbon, and water. From them we can tell what the planetary atmospheres are made of, even from billions of miles away. Not only that, but astronomers are also able to estimate the exoplanet&rsquos temperature. All of these pieces of information provide scientists with clues as to whether or not an exoplanet might harbor life. While astronomers have found chemical signatures for the building blocksof life on other planets (including carbon dioxide, water and methane), so far we have no clear signal of life on another planet.


Some exoplanets seem to have walked directly out of the best science fiction movies. Worlds made of diamond? Volcano covered surfaces? Mysterious magnetic storms? We’ve discovered single exoplanets orbiting two stars and as many as seven exoplanets orbiting a single star. We’ve probed the planet-forming disks that spawn these alien worlds, and we’re even starting to map the weather on these distant planets. Here, you can find the latest exoplanet news, from super-Earths to hot Jupiters. We’ll continue monitoring results from the Kepler and TESS missions and more, as astronomers come up with new and creative methods for studying these alien worlds. We’ll probe the mysteries of nearby systems, report on the farthest-known planets, and share more about their atmospheres and likelihood of habitability.

The hunt for Earth 2.0 is on as we attempt to answer the age-old question: “Are we alone?”


Heller and Armstrong proposed that a series of basic characteristics are required to classify an exoplanet or exomoon as superhabitable [7] [2] [8] [9] [10] for size, it is required to be about 2 Earth masses, and 1.3 Earth radii will provide an optimal size for plate tectonics. [11] In addition, it would have a greater gravitational attraction that would increase retention of gases during the planet's formation. [10] It is therefore likely that they have a denser atmosphere that will offer greater concentration of oxygen and greenhouse gases, which in turn raise the average temperature to optimum levels for plant life to about 25 °C (77 °F). [12] [13] A denser atmosphere may also influence the surface relief, making it more regular and decreasing the size of the ocean basins, which would improve diversity of marine life in shallow waters. [14]

Other factors to consider are the type of star in the system. K-type stars are less massive than the Sun, and are stable on the main sequence for a very long time (18 to 34 billion years, compared to 10 billion for the Sun, a G-class star), [15] [16] giving more time for the emergence of life and evolution. In addition, K-type stars emit less ultraviolet radiation (which can damage DNA and thus hamper the emergence of nucleic acid based life) than G-type stars like the Sun.

Surface, size and composition Edit

An exoplanet with a larger volume than that of Earth, or with a more complex terrain, or with a larger surface covered with liquid water, could be more hospitable for life than Earth. [17] Since the volume of a planet tends to be directly related to its mass, the more massive it is, the greater its gravitational pull, which can result in a denser atmosphere. [18]

Some studies indicate that there is a natural radius limit, set at R, below which nearly all planets are terrestrial, composed primarily of rock-iron-water mixtures. [19] Generally, objects with a mass below 8 M are very likely to be of similar composition as Earth. [20] Above this limit, the density of the planets decreases with increasing size, the planet will become a "water world" and finally a gas giant. [21] [22] In addition, for most super-Earths-with-masses-of-7-times-Earth's, their high mass may cause them to lack plate tectonics. [11] Thus, it is expected that any exoplanet similar to Earth's density and with a radius under 2 R may be suitable for life. [13] However, other studies indicate that water worlds represent a transitional stage between mini-Neptunes and the terrestrial planets, especially if they belong to red dwarfs or K dwarfs. [23] [24] Although water planets may be habitable, the average depth of the water and the absence of land area would not make them superhabitable as defined by Heller and Armstrong. [25] From a geological perspective, the optimal mass of a planet is about 2 M, so it must have a radius that keeps the density of the Earth among 1.2 and 1.3R. [26]

The average depth of the oceans also affects the habitability of a planet. The shallow areas of the sea, given the amount of light and heat they receive, usually are more comfortable for known aquatic species, so it is likely that exoplanets with a lower average depth are more suitable for life. [25] [27] More massive exoplanets would tend to have a regular surface gravity, which can mean shallower—and more hospitable—ocean basins. [28]

Geology Edit

Plate tectonics, in combination with the presence of large bodies of water on a planet, is able to maintain high levels of carbon dioxide ( CO
2 ) in its atmosphere. [29] [30] This process appears to be common in geologically active terrestrial planets with a significant rotation speed. [31] The more massive a planetary body, the longer time it will generate internal heat, which is a major contributing factor to plate tectonics. [11] However, excessive mass can also slow plate tectonics because of increased pressure and viscosity of the mantle, which hinders the sliding of the lithosphere. [11] Research suggests that plate tectonics peaks in activity in bodies with a mass between 1 and 5M, with an optimum mass of approximately 2M. [26]

If the geological activity is not strong enough to generate a sufficient amount of greenhouse gases to increase global temperatures above the freezing point of water, the planet could experience a permanent ice age, unless the process is offset by an intense internal heat source such as tidal heating or stellar irradiation. [32]

Magnetosphere Edit

Another feature favorable to life is a planet's potential to develop a strong magnetosphere to protect its surface and atmosphere from cosmic radiation and stellar winds, especially around red dwarf stars. [33] Less massive bodies and those with a slow rotation, or those that are tidally locked, have a weak or no magnetic field, which over time can result in the loss of a significant portion of its atmosphere, especially hydrogen, by hydrodynamic escape. [11]

Temperature and climate Edit

The optimum temperature for Earth-like life in general is unknown, although it appears that on Earth organism diversity has been greater in warmer periods. [34] It is therefore possible that exoplanets with slightly higher average temperatures than that of Earth are more suitable for life. [35] The thermoregulatory effect of large oceans on exoplanets located in a habitable zone may maintain a moderate temperature range. [36] [35] In this case, deserts would be more limited in area and would likely support habitat-rich coastal environments. [35]

However, studies suggest that Earth already lies near to the inner edge of the habitable zone of the Solar System, [37] and that may harm its long-term livability as the luminosities of main-sequence stars steadily increase over time, pushing the habitable zone outwards. [38] [39] Therefore, superhabitable exoplanets must be warmer than Earth, yet orbit further out than Earth does and closer to the center of the system's habitable zone. [40] [41] This would be possible with a thicker atmosphere or with a higher concentration of greenhouse gases. [42] [43]

Star Edit

The star's type largely determines the conditions present in a system. [45] [46] The most massive stars O, B, and A have a very short life cycle, quickly leaving the main sequence. [47] [48] In addition, O-type stars produce a photoevaporation effect that prevents the accretion of planets around the star. [49] [50]

On the opposite side, the less massive M-and K-types are by far the most common and long-lived stars of the universe, but their potential for supporting life is still under study. [45] [50] Their low luminosity reduces the size of the habitable zone, which are exposed to ultraviolet radiation outbreaks that occur frequently, especially during their first billion year of existence. [15] When a planet's orbit is too short, it can cause tidal locking of the planet, where it always presents the same hemisphere to the star, known as day hemisphere. [51] [50] Even if the existence of life were possible in a system of this type, it is unlikely that any exoplanet belonging to a red dwarf star would be considered "superhabitable". [45]

Dismissing both ends, systems with a K-type stars offer the best habitable zones for life. [15] [50] K-type stars allow the formation of planets around them, have a long life expectancy, and provide a stable habitable zone free of the effects of excessive proximity to its star. [50] Furthermore, the radiation produced by a K-type star is low enough to allow complex life without the need for an atmospheric ozone layer. [15] [52] [53] They are also the most stable and their habitable zone does not move very much during its lifetime, so a terrestrial analog located near a K-type star may be habitable for almost all of the main sequence. [15]

Orbit and rotation Edit

Experts have not reached a consensus about what the optimal rotation speed for an exoplanet is, but it can't be too fast or slow. The latter case can cause problems similar to those observed in Venus, which completes one rotation every 243 Earth days, and as a result, cannot generate an Earth-like magnetic field. A more massive slow-rotation-planet could overcome this problem by having multiple moons due to its higher gravity that can boost the magnetic field. [54] [55]

Ideally, the orbit of a superhabitable world would be at the midpoint of the habitable zone of its star system. [56] [42]

Atmosphere Edit

There are no solid arguments to explain if Earth's atmosphere has the optimal composition to host life. [42] On Earth, during the period when coal was first formed, atmospheric oxygen ( O
2 ) levels were up to 35%, and coincided with the periods of greatest biodiversity. [57] So, assuming that the presence of a significant amount of oxygen in the atmosphere is essential for exoplanets to develop complex life forms, [58] [42] the percentage of oxygen relative to the total atmosphere appears to limit the maximum size of the planet for optimum superhabitability and ample biodiversity [ clarification needed ] .

Also, the atmospheric density should be higher in more massive planets, which reinforces the hypothesis that super-Earths can provide superhabitable conditions. [42]

Age Edit

In a biological context, older planets than Earth may have greater biodiversity, since native species have had more time to evolve, adapt and stabilize the environmental conditions to sustain a suitable environment for life that can benefit their descendants. [16]

However, for many years it was thought that since older star systems have lower metallicity, they should display low planet formation, and thus such old planets may have been scant in the beginning, [59] but the number of metallic items in the universe must have grown steadily since its inception. [60] The first exoplanetary discoveries, mostly gas giants orbiting very close to their stars, known as Hot Jupiters, suggest that planets were rare in systems with low metallicity, which invited suspicion of a time limit on the appearance of the first objects landmass. [61] [ clarification needed ] Later, in 2012, the Kepler telescope's observations allowed experts to find out that this relationship is much more restrictive in systems with Hot Jupiters, and that terrestrial planets could form in stars of much lower metallicity, to some extent. [60] It is now thought that the first Earth-mass objects should appear sometime between 7 and 12 billion years. [60] Given the greater stability of the orange dwarfs (K-type) compared to the Sun (G-type) and longer life expectancy, it is possible that superhabitable exoplanets belonging to K-type stars, orbiting within its habitable zone, could provide a longer, steadier, and better environment for life than Earth. [15]

Despite the scarcity of information available, the hypotheses presented above on superhabitable planets can be summarized as a preliminary profile, even if there is no scientific consensus. [10]

  • Mass: approximately 2M.
  • Radius: to maintain a similar density to Earth, its radius should be close to 1.2 or 1.3R.
  • Oceans: percentage of surface area covered by oceans should be Earth-like but more distributed, without large continuous land masses. The oceans should be shallow the light then will penetrate easier through the water and will reach the fauna and flora, stimulating an abundance of life down in the ocean.
  • Distance: shorter distance from the center of the habitable zone of the system than Earth.
  • Temperature: average surface temperature of about 25 °C (77 °F). [12]
  • Star and age: belonging to an intermediate K-type star with an older age than the Sun (4.5 billion years) but younger than 7 billion years.
  • Atmosphere: somewhat denser than Earth's and with a higher concentration of oxygen. That will make life larger and more abundant.

There is no confirmed exoplanet that meets all these requirements. After updating the database of exoplanets on 23 July 2015, the one that comes closest is Kepler-442b, belonging to an orange dwarf star, with a radius of 1.34R and a mass of 2.36M, but with an estimated surface temperature of 4 °C (39 °F). [62] [63]

Appearance Edit

The appearance of a superhabitable planet should be, in general, very similar to Earth. [64] The main differences, in compliance with the profile seen previously, would be derived from its mass. Its denser atmosphere may prevent the formation of ice sheets as a result of lower thermal difference between different regions of the planet. [42] A superhabitable world would also have a higher concentration of clouds, and abundant rainfall.

The vegetation of such a planet would be very different due to the increased air density, precipitation, temperature, and stellar flux compared to Earth. As the peak wavelength of light differs for K-type stars compared to the Sun, plants may be a different colour than the green vegetation present on Earth. [1] [65] Plant life would also cover more of the surface of the planet, which would be visible from space. [64]

In general, the climate of a superhabitable planet would be warm, moist, homogeneous and have stable land, allowing life to extend across the surface without presenting large population differences in contrast to Earth, which has inhospitable areas such as glaciers, deserts and some tropical regions. [35] If the atmosphere contains enough oxygen, the conditions of these planets may be bearable to humans even without the protection of a space suit, provided that the atmosphere does not contain excessive toxic gases, but they would need to develop adaptations to the increased gravity, such as an increase in muscle and bone density. [64] [24] [66]

Heller and Armstrong speculate that the number of superhabitable planets around Kepler 442-like stars can far exceed that of Earth analogs: [67] less massive stars in the main sequence are more abundant than the larger and brighter stars, so there are more orange (K) dwarfs than solar analogues. [68] It is estimated that about 9% of stars in the Milky Way are K-type stars. [69]

Another point favoring the predominance of superhabitable planets in regard to Earth analogs is that, unlike the latter, most of the requirements of a superhabitable world can occur spontaneously and jointly simply by having a higher mass. [70] A planetary body close to 2 or 3M should have longer-lasting plate tectonics and also will have a larger surface area in comparison to Earth. [10] Similarly, it is likely that its oceans are shallower by the effect of gravity on the planet's crust, its gravitational field more intense and, a denser atmosphere. [12]

By contrast, Earth-mass planets may have a wider range of conditions. For example, some may sustain active tectonics for a shorter time period and will therefore end up with lower air density than Earth, increasing the probability of developing global ice coverage, or even a permanent Snowball Earth scenario. [42] Another negative effect of lower atmospheric density can be manifested in the form of thermal oscillations, which can lead to high variability in the global climate and increase the chance for catastrophic events. In addition, by having a weaker magnetosphere, such planets may lose their atmospheric hydrogen by hydrodynamic escape easier and become a desert planet. [42] Any of these examples could prevent the emergence of life on a planet's surface. [71] In any case, the multitude of scenarios that can turn an Earth-mass planet located in the habitable zone of a solar analogue into an inhospitable place are less likely on a planet that meets the basic features of a superhabitable world, so that the latter should be more common. [67]

In September 2020, astronomers identified 24 superhabitable planet contenders, from among more than 4000 confirmed exoplanets at present, based on astrophysical parameters, as well as the natural history of known life forms on the Earth. [72]

Exoplanets, Worlds Orbiting Other Stars

For centuries, fictional depictions of planets orbiting other stars have fired our imagination. From the desert world of Arrakis in Dune to the lush jungles of Yoda's planet Dagobah in Star Wars, we humans have been fascinated with the idea of exotic, far-off worlds.

We now know that worlds beyond our solar system—known as exoplanets—do exist. In fact, there are a whole lot of them: Scientists have found over 4,000 exoplanets, and think that most stars have their own solar systems. Some exoplanets are surprisingly similar to fictional worlds we've imagined, while others have turned out to be more exotic than anything we could have dreamed.

Our universe is estimated to have over 100 billion galaxies, each with hundreds of billions of stars. If most stars have one or more planet around them, there may be billions of trillions of planets in the universe.

Every exoplanet discovery teaches us something new about how the universe works. When we just had our own solar system’s 8 planets to study, we had a limited view of what kind of planetary systems are possible in the cosmos. Now, with over 4,000 exoplanets cataloged, the horizons of planetary science are broader than ever. We also sometimes get to see other solar systems forming, which teaches us about our own origins. It's like watching our very own "How It's Made" show through the world's telescopes.

There are lots of reasons to learn about exoplanets, but perhaps the most compelling is that we could find another world that hosts living organisms. If we discover life beyond Earth, it could change the course of human history. And with continual advancements in exoplanet research, this discovery could happen in your lifetime.

Exoplanet Proxima b This artist’s impression shows the surface of exoplanet Proxima b orbiting the red dwarf star Proxima Centauri—the closest star to Earth. The planet is similar in size to Earth and lies in its star's habitable zone, the not-too-hot, not-too-cold region where temperatures are suitable for liquid water to exist on the surface. Image: ESO/M. Kornmesser

21.5 Exoplanets Everywhere: What We Are Learning

Before the discovery of exoplanets, most astronomers expected that other planetary systems would be much like our own—planets following roughly circular orbits, with the most massive planets several AU from their parent star. Such systems do exist in large numbers, but many exoplanets and planetary systems are very different from those in our solar system. Another surprise is the existence of whole classes of exoplanets that we simply don’t have in our solar system: planets with masses between the mass of Earth and Neptune, and planets that are several times more massive than Jupiter.

Kepler Results

The Kepler telescope has been responsible for the discovery of most exoplanets, especially at smaller sizes, as illustrated in Figure 21.22, where the Kepler discoveries are plotted in yellow. You can see the wide range of sizes, including planets substantially larger than Jupiter and smaller than Earth. The absence of Kepler-discovered exoplanets with orbital periods longer than a few hundred days is a consequence of the 4-year lifetime of the mission. (Remember that three evenly spaced transits must be observed to register a discovery.) At the smaller sizes, the absence of planets much smaller than one earth radius is due to the difficulty of detecting transits by very small planets. In effect, the “discovery space” for Kepler was limited to planets with orbital periods less than 400 days and sizes larger than Mars.

One of the primary objectives of the Kepler mission was to find out how many stars hosted planets and especially to estimate the frequency of earthlike planets. Although Kepler looked at only a very tiny fraction of the stars in the Galaxy, the sample size was large enough to draw some interesting conclusions. While the observations apply only to the stars observed by Kepler, those stars are reasonably representative, and so astronomers can extrapolate to the entire Galaxy.

Figure 21.23 shows that the Kepler discoveries include many rocky, Earth-size planets, far more than Jupiter-size gas planets. This immediately tells us that the initial Doppler discovery of many hot Jupiters was a biased sample, in effect, finding the odd planetary systems because they were the easiest to detect. However, there is one huge difference between this observed size distribution and that of planets in our solar system. The most common planets have radii between 1.4 and 2.8 that of Earth, sizes for which we have no examples in the solar system. These have been nicknamed super-Earths , while the other large group with sizes between 2.8 and 4 that of Earth are often called mini-Neptunes .

What a remarkable discovery it is that the most common types of planets in the Galaxy are completely absent from our solar system and were unknown until Kepler’s survey. However, recall that really small planets were difficult for the Kepler instruments to find. So, to estimate the frequency of Earth-size exoplanets, we need to correct for this sampling bias. The result is the corrected size distribution shown in Figure 21.24. Notice that in this graph, we have also taken the step of showing not the number of Kepler detections but the average number of planets per star for solar-type stars (spectral types F, G, and K).

We see that the most common planet sizes of are those with radii from 1 to 3 times that of Earth—what we have called “Earths” and “super-Earths.” Each group occurs in about one-third to one-quarter of stars. In other words, if we group these sizes together, we can conclude there is nearly one such planet per star! And remember, this census includes primarily planets with orbital periods less than 2 years. We do not yet know how many undiscovered planets might exist at larger distances from their star.

To estimate the number of Earth-size planets in our Galaxy, we need to remember that there are approximately 100 billion stars of spectral types F, G, and K. Therefore, we estimate that there are about 30 billion Earth-size planets in our Galaxy. If we include the super-Earths too, then there could be one hundred billion in the whole Galaxy. This idea—that planets of roughly Earth’s size are so numerous—is surely one of the most important discoveries of modern astronomy.

Planets with Known Densities

For several hundred exoplanets, we have been able to measure both the size of the planet from transit data and its mass from Doppler data, yielding an estimate of its density. Comparing the average density of exoplanets to the density of planets in our solar system helps us understand whether they are rocky or gaseous in nature. This has been particularly important for understanding the structure of the new categories of super-Earths and mini-Neptunes with masses between 3–10 times the mass of Earth. A key observation so far is that planets that are more than 10 times the mass of Earth have substantial gaseous envelopes (like Uranus and Neptune) whereas lower-mass planets are predominately rocky in nature (like the terrestrial planets).

Figure 21.25 compares all the exoplanets that have both mass and radius measurements. The dependence of the radius on planet mass is also shown for a few illustrative cases—hypothetical planets made of pure iron, rock, water, or hydrogen.

At lower masses, notice that as the mass of these hypothetical planets increases, the radius also increases. That makes sense—if you were building a model of a planet out of clay, your toy planet would increase in size as you added more clay. However, for the highest mass planets (M > 1000 MEarth) in Figure 21.25, notice that the radius stops increasing and the planets with greater mass are actually smaller. This occurs because increasing the mass also increases the gravity of the planet, so that compressible materials (even rock is compressible) will become more tightly packed, shrinking the size of the more massive planet.

In reality, planets are not pure compositions like the hypothetical water or iron planet. Earth is composed of a solid iron core, an outer liquid-iron core, a rocky mantle and crust, and a relatively thin atmospheric layer. Exoplanets are similarly likely to be differentiated into compositional layers. The theoretical lines in Figure 21.25 are simply guides that suggest a range of possible compositions.

Astronomers who work on the complex modeling of the interiors of rocky planets make the simplifying assumption that the planet consists of two or three layers. This is not perfect, but it is a reasonable approximation and another good example of how science works. Often, the first step in understanding something new is to narrow down the range of possibilities. This sets the stage for refining and deepening our knowledge. In Figure 21.25, the two green triangles with roughly 1 MEarth and 1 REarth represent Venus and Earth. Notice that these planets fall between the models for a pure iron and a pure rock planet, consistent with what we would expect for the known mixed-chemical composition of Venus and Earth.

In the case of gaseous planets, the situation is more complex. Hydrogen is the lightest element in the periodic table, yet many of the detected exoplanets in Figure 21.25 with masses greater than 100 MEarth have radii that suggest they are lower in density than a pure hydrogen planet. Hydrogen is the lightest element, so what is happening here? Why do some gas giant planets have inflated radii that are larger than the fictitious pure hydrogen planet? Many of these planets reside in short-period orbits close to the host star where they intercept a significant amount of radiated energy. If this energy is trapped deep in the planet atmosphere, it can cause the planet to expand.

Planets that orbit close to their host stars in slightly eccentric orbits have another source of energy: the star will raise tides in these planets that tend to circularize the orbits. This process also results in tidal dissipation of energy that can inflate the atmosphere. It would be interesting to measure the size of gas giant planets in wider orbits where the planets should be cooler—the expectation is that unless they are very young, these cooler gas giant exoplanets (sometimes called “cold Jupiters”) should not be inflated. But we don’t yet have data on these more distant exoplanets.

Exoplanetary Systems

As we search for exoplanets, we don’t expect to find only one planet per star. Our solar system has eight major planets, half a dozen dwarf planets, and millions of smaller objects orbiting the Sun. The evidence we have of planetary systems in formation also suggest that they are likely to produce multi-planet systems.

The first planetary system was found around the star Upsilon Andromedae in 1999 using the Doppler method, and many others have been found since then (over 700 as of early 2021). If such exoplanetary system are common, let’s consider which systems we expect to find in the Kepler transit data.

A planet will transit its star only if Earth lies in the plane of the planet’s orbit. If the planets in other systems do not have orbits in the same plane, we are unlikely to see multiple transiting objects. Also, as we have noted before, Kepler was sensitive only to planets with orbital periods less than about 4 years. What we expect from Kepler data, then, is evidence of coplanar planetary systems confined to what would be the realm of the terrestrial planets in our solar system.

By 2021, astronomers gathered data on over 700 such exoplanet systems. Many have only two known planets, but a few have as many as five, and one has eight (the same number of planets as our own solar system). For the most part, these are very compact systems with most of their planets closer to their star than Mercury is to the Sun. The figure below shows one of the largest exoplanet systems: that of the star called Kepler-62 (Figure 21.26). Our solar system is shown to the same scale, for comparison (note that the Kepler-62 planets are drawn with artistic license we have no detailed images of any exoplanets).

All but one of the planets in the K-62 system are larger than Earth. These are super-Earths, and one of them (62d) is in the size range of a mini-Neptune, where it is likely to be largely gaseous. The smallest planet in this system is about the size of Mars. The three inner planets orbit very close to their star, and only the outer two have orbits larger than Mercury in our system. The green areas represent each star’s “habitable zone,” which is the distance from the star where we calculate that surface temperatures would be consistent with liquid water. The Kepler-62 habitable zone is much smaller than that of the Sun because the star is intrinsically fainter.

With closely spaced systems like this, the planets can interact gravitationally with each other. The result is that the observed transits occur a few minutes earlier or later than would be predicted from simple orbits. These gravitational interactions have allowed the Kepler scientists to calculate masses for the planets, providing another way to learn about exoplanets.

Kepler has discovered some interesting and unusual planetary systems. For example, most astronomers expected planets to be limited to single stars. But we have found planets orbiting close double stars, so that the planet would see two suns in its sky, like those of the fictional planet Tatooine in the Star Wars films. At the opposite extreme, planets can orbit one star of a wide, double-star system without major interference from the second star.

Planetary system orientation & the search for exoplanets - Astronomy

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