Exoplanets

An exoplanet is a planet orbiting a star other than the sun—hence another term for it, “extrasolar planet”. Until the launch of the Hubble Telescope in 1990 scientists had yet to discover one. Since operations began Hubble has become the pioneer in the study of these far-off worlds. The telescope also settled once and for all the question of the age of the universe. Astronomers thought it was older than 10 billion years, but that was about it. Hubble nailed the age down to 13.8 billion years.

Exoplanet Kepler-186f
Artist’s impression of Kepler-186f. Its discovery confirms Earth-size planets exist in the habitable zones of other stars. Image credit: NASA Ames/SETI Institute/JPL-Caltech/T. Pyle
Hubble Space Telescope

The Hubble Space Telescope was launched on 24th April 1990 and for almost three years was unusable. To its horror Nasa realised a serious problem with the large mirror because its edge was flat not curved. The first telescope to be launched into space had blurred vision. Astro mechanics were sent up on 1st December 1993 on Service Mission 1 and succeeded with a clever work-around, leaving the dud mirror in place but installing a series of corrective optics. Hubble uses four cameras to combine black and white images transmitted down to Hubble Control at Nasa’s Goddard Spaceflight Centre where they are processed into a full colour composite. In February 1997 Service Mission 2 upgraded Hubble with an imaging spectrograph. And in March 2002 the Advanced Camera for Surveys was installed, providing optics ten times more powerful. May 2009 saw the last service mission to give the telescope a complete overhaul. Since then it has and will remain on its own.

Hubble travels at 28 000 kilometres an hour at an altitude of 545 kilometres to complete an orbit of Earth every 97 minutes.

Since Hubble started the ball rolling astronomers have turned their attentions towards deep space in search of planets around other stars.

In 2009 Nasa launched the Kepler Space Telescope with the specific mission of looking for planets that might resemble Earth. Kepler found exoplanets by watching for stars dimming as planets passed in front of them. It was decommissioned in 2018, having racked up some 2 682 potential exoplanets, many since confirmed by additional observations. Candidates put forward by Kepler have an 80%-90% probability to be actual discoveries, according to Nasa, but have yet to be validated.

Exoplanet tally

To date more than 4 000 exoplanets have been discovered and “are considered confirmed”, says Nasa—indeed, the number is 4 152 as of 25th April 2020. But adds that there are thousands of other “candidate” exoplanet detections that require further observations in order to say for sure whether or not the exoplanet is real. Nasa gives a figure of 5 108 candidates and 3 077 planetary systems.

Confirmed exoplanets have been validated by multiple observations. Of the confirmed 4 152 exoplanets:

  • 1 393 are Neptune-like
  • 1 306 are Gas Giants
  • 1 287 are identified as “Super Earths”
  • 160 Terrestrial
  • 6 Unknown

Methods of detection range from transit (76%), radial velocity (19%), microlensing (2%), and imaging.

In radial velocity astronomers watch for wobbles. Orbiting planets cause stars to wobble in space, changing the colour of the light astronomers observe. The transit method is where astronomers look for shadows cast over a star by an orbiting planet. As a planet passes directly between its star and Earth it dims the star’s light and this can be detected. Gravitational microlensing is where light from a distant star is bent and focused by gravity as a planet passes between a star and Earth.

Hot Jupiters

The first exoplanet of a sun-like star was discovered in October 1995. Named 51 Pegasi b, it is 50 light years away from Earth in the constellation Pegasus. It has an orbital period of 4.23 days and orbits seven times closer to its star than Mercury. It is 47% less massive than Jupiter but 50% larger. It typically demonstrates the class of exoplanets known as “hot Jupiters”.

Until the discovery of exoplanets, our view of planetary formation and star systems was skewed to assume our solar system was the de facto model. How wrong we were. If anything, exoplanets and especially the discovery of Super-Earths reveal just how weird and out of the ordinary our solar system really is. Inside of the orbit of Mercury, the solar system is essentially void. This, as it turns out, is unusual. Super-Earths are between two and eight times the mass of Earth. They have hydrogen-rich atmospheres and sit incredibly close to their stars—so much so that they need high-speed orbits to stay in place.

A good example is Gliese 876d, discovered in 2005 from data gathered by the Keck Observatory in Hawaii. Gliese 876d is almost seven times the mass of Earth and is orbiting at just three million kilometres away from its red dwarf star. It is a rocky planet with a surface temperature of about 314°C. It has an orbital period of just under two days. There are three other planets orbiting this star, but they are much further out. One is Gliese 876b (discovered in 1998), which is a gas giant twice the size of Jupiter orbiting at just 30 million kilometres from the star, with an orbital period of 61 days. This is still closer than the orbit or our own planet Mercury. The two other planets, 876c and 876e also orbit within the orbital distance of Mercury. Astronomers note that this tightly packed configuration is far from rare. Indeed, it appears to be the norm. And, far from being fixed, orbiting as regular as clockwork, in fact planetary systems are chaotic and ever-changing. In their research astronomers have found a very large number of so-called “hot Jupiters” like Gliese 876c, which indicates a far more dynamic behaviour than previously expected from observations of our own solar system. Gas giants, as they clear their orbital paths, encounter various gravitational influences that essentially send them on an inward spiral towards their star. They orbit closer and closer. Astronomers say this seems to be a very common phenomenon judging by the plethora of evidence from the study of exoplanets.

Does this suggest our own Jupiter is heading that way too?

The gas giants orbiting close to their stars typically have surface temperatures of 1 100°C. They are slowly evaporating in the heat, which is why they are called “roasters” or “hot Jupiters”. They are the first exoplanet type to be identified and considered the most common configuration.

Typical of the above model is Gliese 581, a red dwarf star at 20.3 light-years away, in the constellation Libra. It has four rocky planets all closer than the orbit of Mercury. And again, typical of the model, they have very short orbital periods. Gliese 581b is nearest to the star and is 16 times the mass of Earth. Then Gliese 571c has five Earth masses, and furthest out is Gliese 581d with seven Earth masses. It orbits its host star in 66.8 days and lies in the habitable zone, while Gliese 581e completes its orbit in just 3.15 days. Gliese 581e has a mass of about 1.9 times that of Earth. The system is 22 light years away from Earth.

“Gliese” refers to the Gliese Catalogue of Nearby Stars.

Exoplanet types

Exoplanets have so far been categorised into five types, “Hot Jupiters” being the first. The other four:

Hot Neptunes. These are planets with masses similar to that of Neptune and, like hot Jupiters, closely orbit their stars.

Chthonian planets. These were previously Hot Jupiters that have had their outer layers stripped away by radiation and stellar winds, forming a long comet-like tail. The chthonian planet is the hypothetical result of this process, leaving just a rocky core down to as small as an Earth mass. Speculators suggest a terrestrial planet could be the outcome.

Super Earths. These planets have a mass of between five and ten Earth masses. They have a variety of densities and compositions. They may range from rocky planets to gas dwarfs. This class includes ocean planets. Surface conditions will be determined by distance from their star, from semi-molten lava types to deep-frozen ice types.

Carbon and iron planets. Iron-dominated planets are made through impacts accumulating carbon and iron, while ejecting lighter materials from their original mantle. Some astronomers think this may be what happened to our own planet Mercury. Earth of course is dominated, not by iron, but by silicate rocks.

Deep space explorers will no doubt fine-tune, and even expand their categories in line with the constant flow of discoveries.

Notable finds

The nearest known exoplanet to date orbits Proxima Centauri, itself the nearest star to our sun at a distance of 4.24 light-years. The planet is known as Proxima Centauri b. Barnard’s Star is 5.98 light-years distant and has one Earth-sized planet orbiting just outside the habitable zone. Barnard’s was discovered in 1916 and is the fourth closest star outside our solar system.

The “habitable zone” (also referred to as the “Goldilocks Zone”) is a zone of orbit around a star considered hospitable to life, where liquid water can permanently exist on the planet’s surface. This zone depends on the star’s mass and luminosity. The habitable zone around a low-mass star is close to the star, even closer than say our own planet Mercury to the sun. The distance to the habitable zone increases with the mass of the star.

Kepler-22b was the first exoplanet found in the Goldilocks Zone. It was discovered in December 2011 by the Kepler Spacecraft. It has an orbital period of 290 days around the sun-like star Kepler-22. The Kepler Spacecraft was launched 7th March 2009 and its mission was to search for Earth-like planets orbiting other stars using a photometer (Schmidt Telescope). It was named after German astronomer Johannes Kepler (1571-1630). After several orbital manoeuvres the telescope was positioned in an Earth-trailing heliocentric orbit—at perihelion 145.1 million kilometres and aphelion 155.7 million kilometres.

Of course, only planets whose orbits are seen edge-on from Earth can be detected.

Kepler-186f was the first validated Earth-sized planet to orbit a distant star in the habitable zone—a range of distance from a star where liquid water might pool on the planet’s surface. Its status was confirmed on 23rd July 2015. It has an orbital period of 130 days and receives one third the energy Earth receives from the sun. It orbits Kepler-186, a red dwarf star 582 light-years from Earth in the constellation Cygnus.

On 30th October 2018, after nine years in deep space collecting data, NASA announced that Kepler had run out of fuel. The spacecraft was retired in its current, safe orbit, away from Earth.

Exoplanets found a little further out include:

Wolf 359 is a red dwarf flare star, which can brighten dramatically from time to time. It is in the constellation Leo at 7.9 light-years away, and has two exoplanets: Wolf 359b and Wolf 359c.

Lalande 21185 in the constellation Ursa Major is 8.3 light-years away, and the brightest red dwarf observable in the northern hemisphere. It has one planet, Lalande 21185 b, discovered in 2017, which has an orbital period of almost 10 days.

Epsilon Eridani is a sun-like star 10.45 light-years away from Earth (it is in the constellation Eridanus).  It also has one planet, Epsilon Eridani b, discovered in 2000.

Other notable finds include Kepler-69, a G-Type main sequence star in the constellation Cygnus, about 2 700 light-years from Earth. It has two planets. Kepler-69c has a diameter of about one and a half times that of Earth, which orbits its star every 242 days. Kepler-69b is just over twice the size of Earth and races round the star every 13 days.

Kepler-62 has five planets, and two of them, Kepler-62e and Kepler-62f, are within the habitable zone. Kepler-62 is in the constellation Lyra, which is 1 200 light years away from Earth. Kepler-62e orbits its host star once every 122 days and is close to the size of Earth. Kepler-62f orbits its host star once every 267 days and is roughly 40% the size of Earth. It is a rocky planet. Some speculate that these two Super-Earths are water worlds likely to have a global ocean.

A very recent find is Kepler-1649c, found by a team of scientists analysing old data from Nasa’s Kepler Space Telescope. The details were published in 2020. It orbits the M-Type red dwarf star Kepler-1649, about 300 light-years from Earth. The planet is a potentially rocky world that could support liquid water. It has an orbital period of 19.5 days and is classed a “Super-Earth”, with a mass equal to 1.2 Earths.

TrES-4b is one of the largest exoplanets so far found. It was discovered in 2006 and derives its name from the discoverers, the Trans-Atlantic Exoplanet Survey (TrES). It is in the constellation Hercules and is 70% bigger than Jupiter.

This team has also found the darkest planet ever, TrES-2b.

Dark planet
Dark Planet
Dark Planet TrES-2b. Image credit: NASA/JPL-Caltech/T. Pyle

TrES-2b was identified in 2011 as the darkest known planet in the universe, reflecting less than 1% of light incident upon it. It is 750 light-years away in the direction of the constellation Draco. One observer says it is even blacker than coal, although a faint red glow (as of burning embers) has also been detected, believed to be generated by internal heat.

It is bizarre how such a huge planet became so absorbent of all the light that it receives. Scientists are not sure what causes this phenomenon but believe it could be “a chemical we haven’t even thought of yet”.

Scientists speculate that TrES-2b lacks reflective clouds, one factor for low reflectance. The atmosphere is super-heated to more than 980°C by a star just five million kilometres away from it.  It is tidally locked like our moon, such that one side of the planet always faces the star. They propose that light-absorbing chemicals such as vaporised sodium and potassium or gaseous titanium oxide in the planet’s atmosphere could also help explain why it is so dark. TrES-2b is also catalogued as Kepler-1b.

Future

Evidence suggests that about 10 percent of nearby Sun-like stars have planets. Current techniques can be used to estimate the orbital parameters of gas giants around these stars and thereby identify those with stable, distant circular orbits. Within such systems, perhaps a quarter of them, there might lie an inner zone where rocky terrestrial planets have formed. These systems will be the first on the list for examination by the latest dedicated planet-hunting telescopes.

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By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.

Last updated: Monday, 27th April 2020