The milestone of 1,000 confirmed exoplanets was surpassed today after twenty-one years of discoveries. The long-established and well-known Extrasolar Planet Encyclopedia
now lists 1,010 confirmed exoplanets. Not all current exoplanet catalogs list the same numbers as this depends on their particular criteria. For example, the more recent NASA Exoplanet Archive
lists just 919. Nevertheless, over 3,500 exoplanet candidates are waiting for confirmation.
The first confirmed exoplanets were discovered by the Arecibo Observatory in 1992. Two small planets were found around the remnants of a supernova explosion known as a pulsar. They were the surviving cores of former planets or newly formed bodies from the ashes of a dead star. This was followed by the discovery of exoplanets around sun-like stars in 1995 and the beginning of a new era of exoplanet hunting.
Exoplanet discoveries have been full of surprises from the outset. Nobody expected exoplanets around the remnants of a dead star (i.e. PSR 1257+12
), nor Jupiter-size orbiting close to their stars (i.e. 51 Pegasi
). We also know today of stellar systems packed with exoplanets (i.e. Kepler-11
), around binary stars (i.e. Kepler-16
), and with many potentially habitable exoplanets (i.e. Gliese 667C
The discovery of many worlds around others stars is a great achievement of science and technology. The work of scientists and engineers from many countries were necessary to achieve this difficult milestone. However, one thousand exoplanets in two decades is still a small fraction of those expected from the billions of stars in our galaxy. The next big goal is to better understand their properties, while detecting many new ones.
* planet candidate CREDIT: PHL @ UPR AreciboBy Dirk Schulze-Makuch
According to the Exoplanet Catalog
maintained by the Planetary Habitability Laboratory (PHL) of the University of Puerto Rico at Arecibo, the number of confirmed planets outside our own solar system is approaching 1,000, while another 3,500 exoplanets—most of them detected by NASA’s Kepler mission—are yet to be confirmed. We’re not talking only about Jupiter- or Neptune-like gas giants, but also Super-Earths (terrestrial planets several times the mass of Earth) and Earth-size planets.
From this growing list, the PHL, directed by Abel Méndez, has identified the top 12 potentially habitable exoplanets based on an Earth Similarity Index (ESI). Their top choice is Kepler 62e, with an ESI value of 0.83 (an ESI of 1.0 would be a 100 % match with Earth in terms of astronomical parameters). Kepler 62e is a Super-Earth in the Constellation Lyra, with an estimated mass of 3.6 Earth masses and an estimated radius 1.6 times that of Earth. Its surface temperature is estimated at 31°C (88°F) and it is 7 billion years old, significantly older than Earth. It’s also very far away—1,200 light years, meaning that we won’t be visiting it any time soon.
How much like our own world is Kepler 62e, really? We should be careful to distinguish Earth similarity from planetary habitability. In many respects the Moon has very similar astronomical values to Earth, yet we know it’s a dead rock. On the other hand there’s Saturn’s moon Titan, a top candidate for finding primitive extraterrestrial life in our solar system. But Titan couldn’t be more different from Earth—an icy moon with liquid methane/ethane lakes on its surface, a nitrogen-methane atmosphere, and temperatures well below Earth’s arctic regions.
My own favorite candidate for a habitable exoplanet is Gliese 581d. A mere 20 light years from us, it’s #12 on the PHL list. The planet has an estimated mass of about seven Earth masses, with a radius about double Earth’s. Gliese 581d orbits a red dwarf star with an orbital period of 67 days, which is important. Why? A red dwarf has less energy output than our (yellow dwarf) sun, and an orbital period of 67 days would put the planet in a Mars-like orbit in terms of temperature. In our own solar system, Mars is cold and dry today because it was too small to retain a thick atmosphere, internal heating, and a magnetic field. However, if Mars had been a Super-Earth like Gliese 581d, it would surely still be heated from inside, have kept its magnetic field and thick atmosphere, and would likely still have liquid oceans (and perhaps even life!) on its surface.
Gliese 581d’s estimated surface temperature of -37°C (-35°F) should not concern us too much. If aliens were to observe Earth from afar, based just on the amount of incoming solar radiation they would estimate our surface temperature as -18°C (0°F). It’s only due to the greenhouse effect that Earth’s average temperature is actually a benign +15°C (59°F; yes, a limited greenhouse effect can be a good thing!). On a Super-Earth planet such as Gliese 581d, I would expect the effect to be quite a bit stronger than on Earth. And one last thing in this planet’s favor: Gliese 581d is nearly twice as old as Earth, which could have given evolution plenty of time to develop advanced, perhaps even technologically advanced, life forms.
The 'Bright Blue Marble' and the 'Pale Blue Dot' Together
Here are actual satellite images of Earth near the moment and the angle the pictures from Cassini and Messenger were taken from Saturn and Mercury on the Day the Earth Smiled, respectively. High resolution black and white images from the GOES East and Meteosat meteorological satellites were combined with color information from NASA Visible Earth to generate true-color images. Check here for additional details.
Earth taken from orbit and from Saturn on the Day the Earth Smiled (July 19, 2013). Earth from orbit is shown with a little more illuminated area at the moment of the Cassini picture from Saturn. The Moon is also visible to the right of Earth in the image from Saturn. Earth was visible to the naked-eye (+1.9 magnitude) as a dim star at the moment the image was taken from Saturn. This figure makes references to the iconic 'Blue Marble' and 'Pale Blue Dot' images of Earth from space. Credit: PHL @ UPR Arecibo, NASA/JPL-Caltech/Space Science Institute, NERC Satellite Station, Dundee University, Scotland.
Earth taken from orbit and from Mercury on the Day the Earth Smiled (July 19, 2013). Earth from orbit is shown with almost exactly the area and illumination at the moment of the Messenger picture from Mercury. The Moon is also visible to the right of Earth in the image from Mercury. Earth was as bright (-4.8 magnitude) as the maximum brightness of Venus at the moment the image was taken from Mercury. This figure makes references to the iconic 'Blue Marble' and 'Pale Blue Dot' images of Earth from space. Credit: PHL @ UPR Arecibo, NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington, NERC Satellite Station, Dundee University, Scotland.
On July 19, 2013 the NASA Cassini and Messenger spacecrafts took pictures of Earth from Saturn and Mercury, respectively. These photos provide some context as to the approximate appearance of Earth during these pictures as seen from geostationary weather satellites. Click image for larger version. Credit: PHL @ UPR Arecibo, NASA, NERC Satellite Station, Dundee University, Scotland.
Figure 4. Earth from the geostationary weather satellite GOES East on July 19, 2013 at 5 PM EST - 2 PM EDT (21 UTC). Click for high resolution version. Credit: PHL @ UPR Arecibo, NASA, NERC Satellite Station, Dundee University, Scotland.
Figure 5. Earth from the geostationary weather satellite GOES East on July 19, 2013 at 5 PM EST - 2 PM EDT (21 UTC). Click for high resolution version. Credit: PHL @ UPR Arecibo, NASA, NERC Satellite Station, Dundee University, Scotland.
The star Gliese 667C is now the best candidate for harboring habitable worlds.
Our Solar System has only one habitable planet, or maybe two if you count Mars’ past when liquid water was running on its surface. More than one potentially habitable planet per star has been a very rare event in the known stars with planets. The well-known star Gliese 581 might have two and just recently the Kepler Space Telescope discovered two in the star Kepler-62. Now a team of scientists, led by Guillem Anglada of the University of Göttingen, reports the discovery of a star with three potentially habitable worlds.
Gliese 667C is part of a nearby triple system of stars 22 light years away. The team discovered that Gliese 667C has six planets, or even more, with three of them in the habitable zone and not much more massive than Earth. Gliese 667C is now the most interesting object for studying stellar systems with the potential for life.
About one hundred exoplanets, of the 898 detected so far, orbit their star at the right distance to sustain liquid water or the so-called ‘habitable zone’. However, most are big Jupiter-like worlds that are unable anyway to have a surface with liquid water. Only a few probably have the right size to be rocky worlds just like Earth.
Gliese 667C is the fourth known planetary system with at least six confirmed planets (Kepler-11, HD 10180, and HD 40307 also have six planets). Gliese 667C f and e will be added to the Habitable Exoplanets Catalog, now having a dozen objects of interest. Gliese 667C c was already added last year.
Artistic representations of the three potentially habitable planets around the star Gliese 667C as compared with Earth. The planets are shown assuming a rocky composition with surfaces mostly covered by water clouds. Credit:
PHL @ UPR Arecibo.
Figure 2. The Habitable Exoplanets Catalog now list a dozen object of interest as potentially habitable worlds with the addition of two new planets, Gliese 667C e and f (Gliese 667C c was known since early 2012). Credit: PHL @ UPR Arecibo.
Figure 3. Simulated view of the star Gliese 667C from its three orbiting potentially habitable worlds. The Earth image is an unretouched photo of a beach sunset. The size of the star, and the brightness and color of the sky was carefully adjusted in subsequent frames to approximate the same view from each planet. The original photo is from a beach in Puerto Rico. Credit: PHL @ UPR Arecibo.
Figure 4. Gliese 667 is a nearby system of three stars that is easy to locate in the Scorpius constellation. The main two central stars Gliese 667AB are barely visible to the naked eye but easy to spot by binoculars or a small telescope. They are so close together that they appear as a single star. Gliese 667C is far enough from the central stars to be seen as separate star. However, it requires a larger telescope to be seem. Credit: PHL @ UPR Arecibo.
Figure 5. Orbits and approximate relative size of the planets around Gliese 667C (orbits and planets are not to scale with each other). All three potentially habitable planets (c, f, and e) orbit within the 'conservative habitable zone.' Credit: PHL @ UPR Arecibo.
Figure 6. This image shows the relative location of each potentially habitable planet of Gliese 667C with respect to the habitable zone. The location of Earth and Mars are shown for comparison. Note that Earth is not at the center of the habitable zone. Any planet close to the inner edge risk being a super-Venus while a frozen world closer to the outer edge. The star is not to scale with the planets.
Table 1: Planetary properties of the six planets around the star Gliese 667C plus one still unconfirmed (Gliese 667C h). Three of these (e, f, and e) are potentially habitable (highlighted in red). This table includes some estimated values that are not part of the original observations (see table notes below). Table 2 has similar values for the Solar System planets.
Planet Type - according to the PHL's Classification of Exoplanets.
Expected Mass - estimate assuming a probable inclination of 60°.
Expected Radius - estimate from an empirical mass-radius relationship.
Equilibrium Temperature - estimate assuming a bond albedo of 0.3.
Expected Surface Temperature - estimate assuming a scaled Earth-like atmosphere.
Habitable Zone Distance (HZD) - a measure of the orbital location of the planet with respect to the HZ. Planets with HZD values between -1 and +1. are within the HZ.
Earth Similarity Index (ESI) - using the new revised ESI which only considers observed parameters, stellar flux, and radius or mass.
Table 2: Planetary properties of the planets of the Solar System for comparison purposes. Description similar to Table 1.
took radar images of asteroid 1998 QE2 and its moon as the space rock sailed safely past earth. The sequence of images show a dark, cratered asteroid 3.0 kilometers across (1.9 miles) across with a companion moon about 750 meters (2500 feet) in size. Both the asteroid and its moon passed 6 million kilometers (3.8 million miles) from earth. This object has no chance of hitting the Earth, but comes close enough to study with a variety of telescopes, large and small.
Figure 1. Radar images of asteroid 1998 QE2 (bottom) and its satellite (top) on June 6. Each image is a sum of 4, spaced apart by about 10 minutes. Each vertical pixel corresponds to 7.5 meters (25 feet) in range, while horizontal pixels correspond to 0.075 Hz (Doppler shift due to rotation).
Figure 2. Radar image from June 7 (left) and from June 9 (right) showing the satellite moving past the primary. Orbital period is about 32 hours.
Many new potentially habitable worlds waiting to be confirmed
Last week NASA’s Kepler mission added 1,924 new objects of interest
to its list of 2,713 exoplanet candidates. The new data has not been completely analyzed yet and many of these objects might be attributed to non-planetary processes (false-positives). However, this new batch gives a general idea of what new types of planets could be announced in the future.
The Planetary Habitability Laboratory (PHL) did a basic analysis of this new Kepler data compared to the existing exoplanets data. The new data suggests the addition of up to 83 new potentially habitable exoplanet candidates, a big increase from the current 18 listed in the Habitable Exoplanets Catalog
. Only four of these have been confirmed so far: Kepler-22b, Kepler-62e, Kepler-62f, and Kepler-61b.
Two types of potential exoplanets are particularly notable in this new Kepler data. There is a big increase of the number of warm and cold super-Earths. This is somewhat expected since the new data includes objects with longer periods. However, other larger objects did not increase as well but it might be too early to attribute this to the abundance of low mass planets.
The most interesting additions are the first potential Earth-like planets. All currently known potentially habitable exoplanets are quite larger than Earth and therefore not very Earth-like by definition. The new Kepler data suggests six new objects with the right size and distance from their star to be considered Earth-like worlds. Since Kepler is no longer operating, it will be very hard to confirm any of these objects with additional data in the near future.
The recent study by Everett et al. (2013)
suggests that one fourth of the Kepler exoplanets candidates are 35% larger than expected, which could also affect the number of those considered potentially habitable. The new Kepler data is being used by the PHL and other groups to prioritize targets of interest for analyses and future observations.
About the Habitable Exoplanets Catalog
The PHL’s Habitable Exoplanets Catalog (HEC) uses a wide definition for potentially habitable exoplanets that includes planets with 0.1 to 10 Earth masses (or 0.4 to 2.6 Earth radii) orbiting the habitable zone
of their parent star within the Venus-Mars empirical limits. Other research groups use narrower or wider definitions. A much wider definition that includes ‘dry planets’ and ‘hydrogen-rich planets’
will be implemented in the future.
New 1,924 NASA Kepler objects of interest added to the NASA Exoplanet Archive. These are objects that are still being considered for inclusion as exoplanets candidates. Notable additions are many 'warm and cold superterrans' (super-Earths) and six 'warm terrans' (potential Earth-like worlds). Check the Periodic Table of Exoplanets
for confirmed and current Kepler exoplanets candidates for comparison. CREDIT: PHL @ UPR Arecibo
Figure 2. Planetary radius versus habitable zone location for the current 890 confirmed exoplanets. All known potentially habitable exoplanets are superterrans (labeled) and there are no terrans yet. Solar System planets in dark blue. Note that most of these radii are estimates based on mass-radius relationship since only their mass was available. CREDIT: PHL @ UPR Arecibo.
FIgure 3. Planetary radius versus habitable zone location for the current 2,713 NASA Kepler exoplanet candidates. All potentially habitable exoplanet candidates are superterrans and there are no terrans yet. Light blue dots are candidates, red dots confirmed, and Solar System planets in dark blue. CREDIT: PHL @ UPR Arecibo.
FIgure 4. Planetary radius versus habitable zone location for the new 1,924 NASA Kepler objects of interest. All potentially habitable objects include many superterrans and now, for the first time, six terrans. Solar System planets in dark blue. CREDIT: PHL @ UPR Arecibo.
Astronomers led by Sarah Ballard from the University of Washington announced the discovery of Kepler-61b, a superterran exoplanet orbiting near the inner edge of the habitable zone of a small K star. Kepler-61b has a size of 2.15 Earth radii and an orbital period of 60 days. It was previously listed as KOI-1361.01 in the NASA Kepler candidates. Kepler-61b receives about 32% more light from its star than Earth from the Sun. Its surface temperature might be close to 40°C assuming an Earth-like atmosphere. Kepler-61b was added to the Habitable Exoplanet Catalog (HEC) and is now ranked number six based on its Earth Similarity Index. HEC lists now ten objects of interest for the search for life outside our Solar System.
Figure 1: Current list of potentially habitable exoplanets including Kepler-61b. Earth, Mars, Jupiter, and Neptune were added for scale.
Figure 2: Current list of potentially habitable exoplanets including Kepler-61b. Earth and Mars were added for scale.
Orbit of Kepler-61b around its parent K-star Kepler-61. The shaded region corresponds to the size of the narrow habitable zone (darker green) and wider habitable zone (lighter green).
Two new exoplanets discovered by NASA Kepler, Kepler-62e and Kepler-62f, were added to the Habitable Exoplanets Catalog. The two planets are part of a planetary systems of five planets around a star smaller than the Sun 1,200 light years away from Earth. Both are considered potentially habitable because they have a size not much larger than Earth and orbit within the habitable zone of their parent star. Kepler-62e is now the most Earth-like exoplanet discovered so far based on the similarity to some of its properties to Earth. However, the actual habitability of Kepler-62e and Kepler-62f depends on conditions that we can not measure yet. Another interesting planet, Kepler-69c, was also announced today but was not included in the catalog because it barely matches our habitability criteria.
Figure 1. Current potentially habitable exoplanets showing the new Kepler-62e and Kepler-62f. Kepler-62e is now the best candidate for an Earth-like planet based on measured parameters. However, the actual potential for life of any of these worlds depends on their atmospheric properties which are unknown at this time.
Figure 2. Current potentially habitable exoplanets showing the new additions, Kepler-62e and Kepler-62f.
Figure 3. Artistic representations of Kepler-62e and Kepler-62f compared with Earth. Both are shown with a similar terrestrial atmosphere thus making Kepler-62e slightly hotter than Earth and Kepler-62f a cold but still habitable world. Another possibility is that both have dense atmospheres. In that case the temperatures of Kepler-62f might be more suitable for life than on Kepler-62e.
Figure 4. Comparison of the orbit and size of the exoplanets of Kepler-62 with the terrestrial planets of our Solar Systems. The darker green shaded area corresponds to the 'conservative habitable zone' while its lighter borders to its 'optimistic habitable zone' extension. Planet sizes and orbits are not to scale between them.
Table 1: Planetary properties of the planets around the K2V star Kepler-62. Kepler-62e and Kepler-62f are potentially habitable and were added to the Habitable Exoplanets Catalog. This table also includes some estimated values that are not part of the original observations. They are described in the table notes below.
Planet Type - according to the PHL's Classification of Exoplanets.
Mass was estimated for comparison purposes only using a mass-radius relationship for rocky planets.
Equilibrium temperature was estimated assuming a bond albedo of 0.3.
Surface temperature was estimated assuming an Earth-like atmosphere.
The habitable zone distance (HZD) is a measure of the location of the planet with respect to the HZ. Planets with HZD values between -1 and +1. are within the HZ.
This table uses the new revised Earth Similarity Index (ESI) which only considers the observed parameters size of the planet and stellar flux.
Table 2: Planetary properties of the terrestrial planets of the Solar System to be used for comparison with those of the planets of Kepler-62. See Table 1.
A New Approach to Search for Earth-like Worlds
The Planetary Habitability Laboratory (PHL) is now searching for Earth-like worlds. The PHL maintains the Habitable Exoplanets Catalog (HEC) in which exoplanets discoveries are classified and compared according to different habitability metrics. Previously the PHL has not been involved in making the initial exoplanet discoveries. Now, the PHL is using new algorithms based on pattern recognition to search for Earth-like worlds within the NASA Kepler Telescope data.
The idea of an Earth-like planet might suggest a world with oceans, breathable air, or even life. However, astronomers are far from getting this information because exoplanets are very far away for our current instruments. For astronomers the definition of Earth-like planets is limited by what we can measure now, their orbit and size. Any exoplanet around a Sun-like star with a similar orbit and size as Earth is considered an Earth-like world.
The new PHL’s project, Search for Potentially Habitable Exoworlds Resembling Earth (SPHERE), is exploring the NASA Kepler data for Earth-like exoplanets and exomoons. Since October 2012 the NASA Kepler data is publicly available for any research team to explore. Many other teams are using this data to discover exoplanets, including the NASA Kepler Team and scientific community projects such as the Hunt for Exomoons with Kepler
and Planet Hunters
Only a few of the nearly 900 confirmed exoplanets barely fit as candidates for potentially habitable worlds. All of these are superterran worlds (super-Earths) up to two times larger than Earth. Scientists are not confident of how habitable these larger worlds might be as compared to Earth. They are more interested in terran worlds (Earth-size) orbiting in the habitable zone of their star, which are more comparable to Earth.
A potentially habitable exoplanet is not necessarily the same as an Earth-like exoplanet. In astrobiology a habitable exoplanet refers to any world suitable for surface or subsurface life of any type, as we know it. This extends to a series of possibilities that are viable for the most environmentally stress tolerant life forms (i.e extremophilic life) but not necessarily by most complex life such as plants, animals, and humans. In theory, an Earth-like planet might be viable for most terrestrial surface life, but future ground and space observatories are needed to confirm the actual habitability of any of such bodies.
The main tool of the SPHERE project is the transit detection software package K-SPHERE. It consists of a series of IDL software tools designed to recognize the faint signals of potential habitable worlds out of the Kepler data using different pattern recognition algorithms. K-SPHERE has been successfully tested identifying previously known confirmed and candidate exoplanets. It is also identifying new signals that are currently under study.
Pattern recognition techniques are widely used in many scientific, engineering, and computer science applications to perform the ‘most likely’ matching of a pattern to a given input. Common day experiences like recognizing faces, understanding words, and reading characters are natural examples of this process. One of its most popular computer applications is in biometrics where a computer is able to distinguish the subtle differences between two similar faces.
Identifying an Earth-like world in the Kepler data is not much harder mathematically than identifying a face in a picture. Consumer cameras do that all the time even matching persons. The problem is that as humans and computers are sometimes confused with similar faces, even with inanimate objects (pareidolia), the signals of Earth-like worlds can be confused with other unrelated stellar, planetary, or instrumental phenomena. It is very challenging to discern false-positives from the Kepler data at these scales.
The SPHERE project is optimized to search for Earth-size exoplanets and exomoons within the habitable zone of stars. It takes advantage of similarity indices such as the Earth Similarity Index (ESI), a measure of Earth-likeness, to optimize the search for interesting targets within all the Kepler data. SPHERE expects to sort out any Earth-like world hopefully within this year, but it takes a lot of additional effort to validate any discovery.
Thanks to the NASA Kepler Telescope, astronomers are not only searching for another ‘Pale Blue Dot,’ as Carl Sagan used to refer to Earth, but more likely for another ‘Pale Blue Sphere,’ since Kepler reveals the size of these worlds. Earth-like worlds are not anymore a dot in space but featureless ‘spheres’. The surface color and features of these ‘spheres’ are a problem for future observatories.
SPHERE is a project of the PHL of the University of Puerto Rico at Arecibo (UPR Arecibo) with the international collaboration of scientists from other institutions. The NASA Kepler Telescope is a statistical mission that uses the transit method to detect exoplanets, especially those terrestrial ones in the habitable zone of their stars where liquid water and possibly life might exist.
Caption: Simulated transit to scale of some of the planets of the Solar System as seem from far away. Earth with the Moon (center left of the Sun) and Saturn with Titan (center right from the Sun) are show transiting the Sun. Note that Earth and the moons are about the size of many of the sunspots thus making them harder to recognize from the Sun's background. Jupiter (left) and Neptune (right) are also shown to scale but not transiting. CREDIT: PHL @ UPR Arecibo, NASA, A. Fuji.
The number of potentially habitable exoplanets will be impacted
A team of astronomers from Penn State led by Ravi Kumar Kopparapu and Ramses Ramírez, also PHL collaborators, announced a redefinition of the classical stellar habitable zone. Their study has strong implications on how we search and study potentially habitable exoplanets.
The main conclusion of their study is that the habitable zones are actually farther away from the stars than previously thought. Interestingly, Earth appears to be now situated at the very inner edge of the habitable zone.
The new study builds on the previous work of James Kasting, Evan Pugh Professor of Geosciences at Penn State, also one of the co-authors. Currently, all definitions of the habitable zone were based on Dr. Kasting's pioneering work.
Estimates with the new habitable zone suggest that some exoplanets previously believed to be in the habitable zone may not be. However, the study has not as yet taken into account the complicated feedback of clouds, which also help to stabilize climates.
The PHL’s Habitable Exoplanet Catalog (HEC) will certainly be impacted by the new definition of the habitable zone. This will affect the classification of a number of potentially habitable exoplanets out of those nearly 900 confirmed and the over 2,700 NASA Kepler exoplanets candidates.
Recently, the Earth Similarity Index (ESI), a measure of Earth-likeness, and other habitability metrics were updated at the PHL. The ESI does not rely on the definition of the habitable zone but other considerations were used to improve it.
A major release of HEC, reflecting all these results, will be presented via a public lecture at the Arecibo Observatory on Saturday, February 16, 2013 @ 6 PM AST (5 PM EST) and available at the PHL’s site on Monday, February 18, 2013 @ 9 AM AST (8 AM EST).
Figure 1: Extension of the new habitable zone as a function of distance for cool to hot stars. Credit: Chester Harman, Penn State
Extension of the new habitable zone as a function of stellar flux (relative to Earth 100%) for cool to hot stars. Credit: Chester Harman, Penn State