Artist's impression of the surface of a hycean world. Hycean worlds are still hypothetical, and have large oceans and thick hydrogen-rich atmospheres that trap heat. They could be habitable even if they're outside the traditional habitable zone. Credit: University of Cambridge
The search for planets beyond our Solar System (extrasolar planets) has grown by leaps and bounds in the past decade. A total of 4,514 exoplanets have been confirmed in 3,346 planetary systems, with another 7,721 candidates awaiting confirmation. At present, astrobiologists are largely focused on the “low hanging fruit” approach of looking for exoplanets that are similar in size, mass, and atmospheric composition to Earth (aka. “Earth-like.”)
However, astrobiologists are also interested in finding examples of “exotic life,” the kind that emerged under conditions that are not “Earth-like.” For example, a team of astronomers from the University of Cambridge recently conducted a study that showed how life could emerge on ocean-covered planets with hydrogen-rich atmospheres (aka. “Hycean” planets). These findings could have significant implications for exoplanet studies and the field of astrobiology.
Researchers at the Flatiron Institute’s Center for Computational Astrophysics published a paper last week that just might explain a mysterious gap in planet sizes beyond our solar system. Planets between 1.5 and 2 times Earth’s radius are strikingly rare. This new research suggests that the reason might be because planets slightly larger than this, called mini-Neptunes, lose their atmospheres over time, shrinking to become ‘super-Earths’ only slightly larger than our home planet. These changing planets only briefly have a radius the right size to fill the gap, quickly shrinking beyond it. The implication for planetary science is exciting, as it affirms that planets are not static objects, but evolving and dynamic worlds.
In the past two and a half decades, astronomers have confirmed the existence of thousands of exoplanets. In recent years, thanks to improvements in instrumentation and methodology, the process has slowly been shifting from the process of discovery to that of characterization. In particular, astronomers are hoping to obtain spectra from exoplanet atmospheres that would indicate their chemical composition.
This is no easy task since direct imaging is very difficult, and the only other method is to conduct observations during transits. However, astronomers of the CARMENES consortium recently reported the discovery of a hot rocky super-Earth orbiting the nearby red dwarf star. While being extremely hot, this planet has retained part of its original atmosphere, which makes it uniquely suited for observations using next-generation telescopes.
Artists’s impression of the rocky super-Earth HD 85512 b. Credit: ESO/M. Kornmesser
We’ve learned a thing or two about exoplanets in the past several years. One of the more surprising discoveries is that our solar system is rather unusual. The Sun’s worlds are easily divided into small rocky planets and large gas giants. Exoplanets are much more diverse, both in size and composition.
An artist’s conception of the magnetic fields of selected super-Earths as the Z machine, pictured at bottom, mimics the gravitational conditions on other planets. Credits: Eric Lundin; Z photo by Randy Montoya
Deep inside planet Earth, there is a liquid outer core and a solid inner core that counter-rotate with each other. This creates the dynamo effect that is responsible for generating Earth’s planetary magnetic field. Also known as a magnetosphere, this field keeps our climate stable by preventing Earth’s atmosphere from being lost to space. So when studying rocky exoplanets, scientists naturally wonder if they too have magnetospheres.
Unfortunately, until we can measure an exoplanet’s magnetic fields, we are forced to infer their existence from the available evidence. This is precisely what researchers at the Sandia National Laboratories did with its Z Pulsed Power Facility (PPF). Along with their partners at the Carnegie Institution for Science, they were able to replicate the gravitational pressures of “Super-Earths” to see if they could generate magnetic fields.
Several types of exoplanets. Credit: PHL @ UPR Arecibo
The planets in our solar system are broadly divided into two groups: small, rocky worlds like Earth, and large gas giants. Before the discovery of exoplanets, it was thought that our solar system was very typical. The light and heat of a star push the gas to the outer solar system, while heavier dust remains closer to the star. Thus a solar system has close rocky planets and distant gas giants. But we now know that planets and star systems have much more diversity.
Artistic design of the super-Earth GJ 625 b and its star, GJ625 (Gliese 625). Credit: Gabriel Pérez/SMM (IAC)
In the past decade, the study of exoplanets has grown by leaps and bounds. At present, a total of 4,201 planets have been confirmed beyond the Solar System and another 5,481 candidates await confirmation. In the midst of all this, M-type red dwarf stars have become a focus of exoplanet research because they appear to be the most likely place where rocky (aka. Earth-like) planets can be found orbiting within the star’s habitable zone (HZ).
However, that does not mean that red dwarf stars are good candidates for hosting habitable planets. Take GJ 887, for example, one of the brightest M stars in the sky that has a system of two (possibly three) planets. In the past, this star was believed to be calm and stable, but new research by astronomers from Arizona State University has shown that GJ 887 might not be so calm as previously thought.
Artist's conception of HD 21749c, the first Earth-sized planet found by NASA's Transiting Exoplanets Survey Satellite (TESS), as well as its sibling, HD 21749b, a warm sub-Neptune-sized world. Credit: Robin Dienel/Carnegie Institution for Science.
NASA’s new planet-hunting telescope, TESS (Transiting Exoplanet Survey Satellite), just found its first Earth-sized world. Though the Earth-sized planet, and its hot sub-Neptune companion, were first observed by TESS in January 2019, it’s taken until now to confirm their status with ground-based follow-up observations. The discovery is published in The Astrophysical Journal Letters.
Artist's impression of a yellowish star being orbited by an extra-solar planet. Credit: ESO/L. Calçada
It has been an exciting time for the field of exoplanet studies lately! Last summer, researchers from the European Southern Observatory (ESO) announced the discovery of an Earth-like planet (Proxima b) located in the star system that is the nearest to our own. And just six months ago, an international team of astronomers announced the discovery of seven rocky planets orbiting the nearby star TRAPPIST-1.
But in what could be the most encouraging discovery for those hoping to find a habitable planet beyond Earth, an an international team of astronomers just announced the discovery of four exoplanet candidates in the tau Ceti system. Aside from being close to the Solar System – just 12 light-years away – this find is also encouraging because the planet candidates orbit a star very much like our own!
The study that details these findings – “Color difference makes a difference: four planet candidates around tau Ceti” – recently appeared online and has been accepted for publication in the Astrophysical Journal. Led by researchers from the Center for Astrophysics Research (CAR) at the University of Hertfordshire, the team analyzed tau Ceti using a noise-eliminating model to determine the presence of four Earth-like planets.
This illustration compares the four planets detected around the nearby star tau Ceti (top) and the inner planets of our solar system (bottom). Credit: Fabo Feng/CAR/Univ. of Hertfordshire
This discovery was made possible thanks to ongoing improvements in instrumentation, observation and data-sharing, which are allowing for surveys of ever-increasing sensitivity. As Steven Vogt, a professor of astronomy and astrophysics at UC Santa Cruz and a co-author on the paper, said in a UCSC press release:
“We are now finally crossing a threshold where, through very sophisticated modeling of large combined data sets from multiple independent observers, we can disentangle the noise due to stellar surface activity from the very tiny signals generated by the gravitational tugs from Earth-sized orbiting planets.”
This is the latest in a long-line of surveys of tau Ceti, which has been of interest to astronomers for decades. By 1988, several radial velocity measurements were conducted of the star system that ruled out the possibility of massive planets at Jupiter-like distances. In 2012, astronomers from UC Santa Barabara presented a study that indicated that tau Ceti might be orbited by five exoplanets, two of which were within the star’s habitable zone.
The team behind that study included several members who produced this latest study. At the time, lead author Mikko Tuomi (University of Hertfordshire, a co-author on the most recent one) was leading an effort to develop better data analysis techniques, and used this star as a benchmark case. As Tuomi explained, theses efforts allowed them to rule out two of the signals that has previously been identified as planets:
“We came up with an ingenious way of telling the difference between signals caused by planets and those caused by star’s activity. We realized that we could see how star’s activity differed at different wavelengths and use that information to separate this activity from signals of planets.”
Artist’s impression of the Tau Ceti system, based on data retrieved in 2012. Credit: J. Pinfield/Univ. of Hertfordshire
For the sake of this latest study – which was led by Fabo Feng, a member of the CAR – the team relied on data provided by the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph at the ESO’s La Silla Observatory in Chile, and the High Resolution Echelle Spectrometer (HIRES) instrument at the W. M. Keck Observatory in Mauna Kea, Hawaii.
From this, they were able to create a model that removed “wavelength dependent noise” from radial velocity measurements. After applying this model to surveys made of tau Ceti, they were able to obtain measurements that were sensitive enough to detect variations in the star’s movement as small as 30 cm per second. In the end, they concluded that tau Ceti has a system of no more than four exoplanets.
As Tuomi indicated, after several surveys and attempts to eliminate extraneous noise, astronomers may finally have a clear picture of how many planets tau Ceti has, and of what type. “[N]o matter how we look at the star, there seem to be at least four rocky planets orbiting it,” he said. “We are slowly learning to tell the difference between wobbles caused by planets and those caused by stellar active surface. This enabled us to essentially verify the existence of the two outer, potentially habitable planets in the system.”
They further estimate from their refined measurements that these planets have masses ranging from four Earth-masses (aka. “super-Earths”) to as low as 1.7 Earth masses, making them among the smallest planets ever detected around a nearby sun-like star. But most exciting of all is the fact that that two of these planets (tau Ceti e and f) are located within the star’s habitable zone.
Recent studies have shown that rocky planets orbiting red dwarf stars will be tidally-locked and subject to intense radiation, reducing their chances of being habitable. Credit: M. Weiss/CfA
The reason for this is because tau Ceti is a G-type (yellow dwarf) star, which makes it similar to our own Sun – about 0.78 times as massive and half as bright. In contrast, many recently discovered exoplanets – such as Proxima b and the seven planets of TRAPPIST-1 – all orbit M-type (red dwarf) stars. Compared to our Sun, these stars are variable and unstable, increasing their chances of stripping the atmospheres of their respective planets.
In addition, since red dwarfs are much dimmer than our Sun, a rocky planet would have to orbit very closely to them in order to be within their habitable zones. At this kind of distance, the planet would likely be tidally-locked, meaning that one side would constantly be facing towards the sun. This too makes the odds of life emerging on any such planet pretty slim.
Because of this, astronomers have been looking forward to finding more exoplanets around stars that are closer in size, mass and luminosity to our own. But before anyone gets too excited, its important to note these worlds are Super-Earths – with up to four times the mass of Earth. This means that (depending on their density as well) any life that might emerge on these planets would be subject to significantly increased gravity.
In addition, a massive debris disc surrounds the star, which means that these outermost planets are probably subjected to intensive bombardment by asteroids and comets. This not doesn’t exactly bode well for potential life on these planets! Still, this study is very encouraging, and for a number of reasons. Beyond finding strong evidence of exoplanets around a Sun-like star, the measurements that led to their detection are the most sensitive to date.
Artist’s impression of how an infant earth might look. Credit: ESO.
At the rate that their methods are improving, researchers should be getting to the 10-centimeter-per-second limit in no time at all. This is the level of sensitively required for detecting Earth analogs – aka. the brass ring for exoplanet-hunters. As Feng indicated:
“Our detection of such weak wobbles is a milestone in the search for Earth analogs and the understanding of the Earth’s habitability through comparison with these analogs. We have introduced new methods to remove the noise in the data in order to reveal the weak planetary signals.”
Think of it! In no time at all, exoplanet-hunters could be finding a plethora of planets that are not only very close in size and mass to Earth, but also orbiting within their stars habitable zones. At that point, scientists are sure to dispense with decidedly vague terms like “potentially habitable” and “Earth-like” and begin using terms like “Earth-analog” confidently. No more ambiguity, just the firm conviction that Earth is not unique!
With an estimated 100 billion planets in our galaxy alone, we’re sure to find several Earths out here. One can only hope they have given rise to complex life like our own, and that they are in the mood to chat!
The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute
In studying our Solar System over the course of many centuries, astronomers learned a great deal about the types of planets that exist in our universe. This knowledge has since expanded thanks to the discovery of extrasolar planets, many of which are similar to what we have observed here at home.
For example, while hundreds of gas giants of varying size have been detected (which are easier to detect because of their size), numerous planets have also been spotted that are similar to Earth – aka. “Earth-like”. These are what is known as terrestrial planets, a designation which says a lot about a planet how it came to be.
Definition:
Also known as a telluric or rocky planet, a terrestrial planet is a celestial body that is composed primarily of silicate rocks or metals and has a solid surface. This distinguishes them from gas giants, which are primarily composed of gases like hydrogen and helium, water, and some heavier elements in various states.
The term terrestrial planet is derived from the Latin “Terra” (i.e. Earth). Terrestrial planets are therefore those that are “Earth-like”, meaning they are similar in structure and composition to planet Earth.
Artist’s concept for the range of Earth-like extrasolar planets that have been discovered in recent years. Credit: NASA/JPL
Composition and Characteristics:
All terrestrial planets have approximately the same type of structure: a central metallic core composed of mostly iron, with a surrounding silicate mantle. Such planets have common surface features, which include canyons, craters, mountains, volcanoes, and other similar structures, depending on the presence of water and tectonic activity.
Terrestrial planets also have secondary atmospheres, which are generated through volcanism or comet impacts. This also differentiates them from gas giants, where the planetary atmospheres are primary and were captured directly from the original solar nebula.
Terrestrial planets are also known for having few or no moons. Venus and Mercury have no moons, while Earth has only the one (the Moon). Mars has two satellites, Phobos and Deimos, but these are more akin to large asteroids than actual moons. Unlike the gas giants, terrestrial planets also have no planetary ring systems.
The Earth’s interior structure, shown here as consisting of multiple “layers”. Credit: discovermagazine.com
Solar Terrestrial Planets:
All those planets found within the Inner Solar System – Mercury, Venus, Earth and Mars – are examples of terrestrial planets. Each are composed primarily of silicate rock and metal, which is differentiated between a dense, metallic core and a silicate mantle. The Moon is similar, but has a much smaller iron core.
Io and Europa are also satellites that have internal structures similar to that of terrestrial planets. In the case of the former, models of the moon’s composition suggest that the mantle is composed primarily of silicate rock and iron, which surrounds a core of iron and iron sulphide. Europa, on the other hand, is believed to have an iron core that is surrounded by an outer layer of water.
Dwarf planets, like Ceres and Pluto, and other large asteroids are similar to terrestrial planets in the fact that they do have a solid surface. However, they differ in that they are, on average, composed of more icy materials than rock.
Extrasolar Terrestrial Planets:
Most of the planets detected outside of the Solar System have been gas giants, owing to the fact that they are easier to spot. However, since 2005, hundreds of potentially terrestrial extrasolar planets have been found – mainly by the Kepler space mission. Most of these have been what is known as “super-Earths” (i.e. planets with masses between Earth’s and Neptune’s).
Examples of extrasolar terrestrial planets include Gliese 876 d, a planet that has a mass 7 to 9 times that of Earth. This planet orbits the red dwarf Gliese 876, which is located approximately 15 light years from Earth. The existence of three (or possibly four) terrestrial exoplanets was also confirmed between 2007 and 2010 in the Gliese 581 system, another red dwarf roughly 20 light years from Earth.
The smallest of these, Gliese 581 e, is only about 1.9 Earth masses, but orbits very close to the star. Two others, Gliese 581 c and Gliese 581 d, as well as a proposed fourth planet (Gliese 581 g) are more-massive super-Earths orbiting in or close to the habitable zone of the star. If true, this could mean that these worlds are potentially habitable Earth-like planets.
The first confirmed terrestrial exoplanet, Kepler-10b – a planet with between 3 and 4 Earth masses and located some 460 light years from Earth – was found in 2011 by the Kepler space mission. In that same year, the Kepler Space Observatory team released a list of 1235 extrasolar planet candidates, including six that were “Earth-size” or “super-Earth-size” (i.e. less than 2 Earth radii) and which were located within their stars’ habitable zones.
Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range. As of January, 2013, 2740 planet candidates have been discovered.
Categories:
Scientists have proposed several categories for classifying terrestrial planets. Silicate planets are the standard type of terrestrial planet seen in the Solar System, which are composed primarily of a silicon-based rocky mantle and a metallic (iron) core.
Iron planets are a theoretical type of terrestrial planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass. Planets of this type are believed to form in the high-temperature regions close to a star, and where the protoplanetary disk is rich in iron. Mercury is possible example, which formed close to our Sun and has a metallic core equal to 60–70% of its planetary mass.
Coreless planets are another theoretical type of terrestrial planet, one that consists of silicate rock but has no metallic core. In other words, coreless planets are the opposite of an iron planet. Coreless planets are believed to form farther from the star where volatile oxidizing material is more common. Though the Solar System has no coreless planets, chondrite asteroids and meteorites are common.
And then there are Carbon planets (aka. “diamond planets”), a theoretical class of planets that are composed of a metal core surrounded by primarily carbon-based minerals. Again, the Solar System has no planets that fit this description, but has an abundance of carbonaceous asteroids.
Until recently, everything scientists knew about planets – which included how they form and the different types that exist – came from studying our own Solar System. But with the explosion that has taken place in exoplanet discovery in the past decade, what we know about planets has grown significantly.
For one, we have come to understand that the size and scale of planets is greater than previously thought. What’s more, we’ve seen for the first time that many planets similar to Earth (which could also include being habitable) do in fact exist in other Solar Systems.
Who knows what we will find once we have the option of sending probes and manned missions to other terrestrial planets?
Astronomy Cast has episodes on the terrestrial planets including Mars, and an interview with Darin Ragozzine, one of the Kepler Space Mission scientists.
