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  • Writer's pictureSven Piper

Fascinating discoveries

Updated: 7 days ago

Artist's impression of Gliese 667
Artist's impression of Gliese 667 (Credit: ESO/L. Calçada)

Thousands of extrasolar planets have already been discovered, and hundreds are added every year. This special feature deals with the most interesting worlds.

Gliese 581

The Gliese 581 system is one of the most intriguing solar systems. Located 20.3 light-years away in the constellation of Libra, it is among the 100 closest systems. This system has 4 to 6 planets orbiting a red dwarf star, which has only a third of the mass of our sun, although not all planetary discoveries have been confirmed yet.

In April 2007, Gliese 581 c was discovered as the first low-mass planet (with five times the mass of Earth) near the habitable zone. If there is a natural greenhouse effect similar to Earth's, this planet could potentially harbor life. The planet Gliese 581 d, discovered shortly afterward, lies safely within this zone and, with eight times the mass of Earth, is also considered a super-Earth, which is why messages have been sent to this planet from Earth [1].

However, the discovery of Gliese 581 e in April 2009 garnered even more attention, as this planet has only 1.9 times the mass of Earth, making it the most Earth-like planet discovered to date. However, due to its close proximity to the star at only 0.03 AU, this planet lies outside the zone where temperatures would allow for liquid water. Moreover, there is no atmosphere on this planet, as the radiation from the star and solar winds would strip it away. Gliese 581 e completes an orbit around its star in just 3.15 days.

A planet in the habitable zone
A planet in the habitable zone (Credit: ESO)


The TRAPPIST-I system is 39.6 light-years away. Around a cool red dwarf, which has 9 percent of the mass and 12 percent of the radius of our sun, orbit 7 different planets. What makes this system special is that several of these planets are not much larger than Earth, and three of them are firmly within the habitable zone.

In November 2018, researchers from the University of Washington announced that TRAPPIST-I e is very likely an Earth-like world with a possible ocean, making it an excellent candidate for the search for extraterrestrial life [2].

Artist’s impressions of the TRAPPIST-1 planetary system
Artist’s impressions of the TRAPPIST-1 planetary system (Credit: ESO)

HD 209458 b (Osiris)

HD 209458 b, better known by the name of the Egyptian god Osiris, is located 150 light-years from us. This planet has two-thirds the mass of Jupiter but is slightly larger due to the heating of its atmosphere from close proximity to its star. Because it is only 6.92 million km from its star, it completes an orbit in just 3.5 days, categorizing Osiris as a "hot Jupiter."

This planet became famous because its atmosphere is being vaporized by the star, creating a "cometary tail." Additionally, the Hubble Space Telescope has detected significant amounts of water vapor, as well as oxygen and carbon, in the atmosphere of this planet.

PSR B1620-26c

Known as the Methuselah of planets, PSR B1620-26c is 13 billion years old, existing a few billion years longer than our solar system, which is approximately 4.5 billion years old. This Jupiter-sized planet, detected by the Hubble Space Telescope, has an extraordinary history, being located near a burnt-out star and a pulsar in the core of the ancient globular cluster M 4, 5,600 light years away in the constellation of Scorpius [3].

Globular clusters typically have a low proportion of heavy elements, as they formed at a time when the universe was still deficient in these materials. They are considered unlikely places to find extrasolar planets because even gas giants like Saturn and Jupiter have solid metal cores.

The discovery of this planet dates back to 1988, when a pulsar named PSR B1620-26 was found in the globular cluster M 4. This rapidly spinning neutron star emits radio pulses like a lighthouse and is orbited by a so-called "white dwarf." Irregularities in the pulsar's emissions suggested a third body, which pointed to a planet but initially could not be conclusively identified as not a brown dwarf. However, thanks to the Hubble Space Telescope, the mass of this object was determined to be 2.5 times that of Jupiter, confirming it as too light to be a brown dwarf.

This planet's history is notable because it has survived intense ultraviolet radiation, the radiation from a supernova, and its shock wave.

HD 70642 b

Another noteworthy discovery is the planet around the distant star HD 70642. This planet, twice the mass of Jupiter, orbits in an almost circular path, which is rare as most extrasolar planets have very tight or irregular orbits. Its orbital period of six years and its distance from HD 70642, about three-fifths of the distance between the Sun and Jupiter, evoke similarities to our solar system.

This planet was discovered using the Anglo-Australian Telescope in southeastern Australia through the slight wobbling of the sun-like star HD 70642.

How Many Planetary Systems Are There?

Artist’s impression of the TRAPPIST-1 planetary system
Artist’s impression of the TRAPPIST-1 planetary system (Credit: Credit: ESO/N. Bartmann/

Recent estimates about the number of stars in the visible part of the universe suggest that the discoveries of extrasolar planets are extraordinary but not unique. According to scientists at the Australian National University, the sky contains no fewer than 70 trillion stars, although only about 5,400 can be seen with the naked eye.

Many of these stars have planetary systems, according to Simon Driver, one of the scientists involved in this research. Even though life could only develop on a fraction of these planets, the sheer number of potential habitable planets means that scientists continue to search for a second Earth with optimism.

Note: You can find out a lot more about this topic in my book “Exoplaneten - Die Suche nach einer zweiten Erde”, which is in German.

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