This artist's illustration shows the exoplanet SPECULOOS-3 b orbiting its red dwarf star. The planet is as big around as Earth, while its star is slightly bigger than Jupiter – but much more massive. The planet is a prime candidate for follow-up studies with the JWST.
Credit: NASA/JPL-Caltech
Red dwarf stars, also known as M-dwarfs, dominate the Milky Way’s stellar population. They can last for 100 billion years or longer. Since these long-lived stars make up the bulk of the stars in our galaxy, it stands to reason that they host the most planets.
Astronomers examined one red dwarf star named SPECULOOS-3, a Jupiter-sized star about 55 light-years away, and found an Earth-sized exoplanet orbiting it. It’s an excellent candidate for further study with the James Webb Space Telescope.
Observations with the European Southern Observatory's (ESO) Very Large Telescope Interferometer (VLTI) found various silicate compounds and potentially iron, substances we also find in large amounts in the solar system's rocky planets. Credit: Jenry
Researchers using the ESO’s Very Large Telescope Interferometer (VLTI) have found three iron rings around a young star about 500 light-years away. The rings indicate that planets are forming. What can these rings tell us about how Earth and the other planets in our Solar System formed?
This artist's concept illustrates a young, red dwarf star surrounded by three planets. There's growing evidence that red dwarfs place serious limits on exoplanet habitability. Image Credit: By NASA/JPL-Caltech - NASA Image of the Day, Public Domain, https://commons.wikimedia.org/w/index.php?curid=17104843
Red dwarf exoplanet habitability is a hot topic in space science. These small dim stars host lots of exoplanets, including small rocky ones the size of Earth. But the little stars emit extremely powerful flares that can damage and strip away atmospheres.
If we’re ever going to understand red dwarf habitability, we need to understand the atmospheres of the exoplanets that orbit them.
This is an artist’s concept of the nearby exoplanet, LTT 1445Ac, which is a nearby Earth-size world. The planet orbits a red dwarf star. The star is in a triple system, with two closely orbiting red dwarfs seen at upper right. The black dot in front of the foreground star is planet LTT 1445Ab, transiting the face of the star. Image Credit: NASA, ESA, L. Hustak (STScI)
Twenty-two light-years away, a rocky world orbits a red dwarf. It’s called LTT 1445Ac, and NASA’s Transiting Exoplanet Survey Satellite (TESS) found it in 2022. However, TESS was unable to gauge the small planet’s size.
That’s okay. The venerable Hubble took care of it.
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.
In the past few decades, the number of planets discovered beyond our Solar System has grown exponentially. To date, a total of 4,158 exoplanets have been confirmed in 3,081 systems, with an additional 5,144 candidates awaiting confirmation. Thanks to the abundance of discoveries, astronomers have been transitioning in recent years from the process of discovery to the process of characterization.
In particular, astronomers are developing tools to assess which of these planets could harbor life. Recently, a team of astronomers from the Carl Sagan Institute (CSI) at Cornell University designed an environmental “decoder” based on the color of exoplanet surfaces and their hosts stars. In the future, this tool could be used by astronomers to determine which exoplanets are potentially-habitable and worthy of follow-up studies.
Illustration of Kepler-186f, a recently-discovered, possibly Earthlike exoplanet that could be a host to life. Scientists could use this one or one like it to measure planetary entropy production as a prelude to exploration. (NASA Ames, SETI Institute, JPL-Caltech, T. Pyle)
To date, astronomers have confirmed the existence of 4,144 extrasolar planets in 3,074 systems, with a further 5,094 candidates awaiting confirmation. The majority of these planets were found by the Kepler Space Telescope, which spent nine years (between May of 2009 and February of 2018) monitoring distant stars for transit signals – where a planet passing in front of a star causes a dip in brightness.
And yet, even though it is now defunct, the data that Kepler accumulated over the years continues to lead to new discoveries. For instance, a transatlantic team of researchers recently found a signal in Kepler‘s archival data that eluded detection before. This signal indicates that there is a second planet orbiting Kepler-1649, an M-type red dwarf star located 302 light-years away.
An asteroid torn apart by the strong gravity of a white dwarf has formed a ring of dust particles and debris orbiting the Earth-sized burnt out stellar core. Image Credit: University of Warwick/Mark Garlick
Sad fact of the Universe is that all stars will die, eventually. And when they do, what happens to their babies? Usually, the prognosis for the planets around a dying star is not good, but a new study says some might in fact survive.
A group of astronomers have taken a closer look at what happens when stars, like our Sun for instance, become white dwarfs late in their lives. As it turns out, denser planets like Earth might survive the event. But, only if they’re the right distance away.
Most exoplanets orbit red dwarf stars because they're the most plentiful stars. This is an artist's illustration of what the TRAPPIST-1 system might look like from a vantage point near planet TRAPPIST-1f (at right). Credits: NASA/JPL-Caltech
In February of 2017, NASA scientists announced the existence of seven terrestrial (i.e. rocky) planets within the TRAPPIST-1 star system. Since that time, the system has been the focal point of intense research to determine whether or not any of these planets could be habitable. At the same time, astronomers have been wondering if all of the system’s planets are actually accounted for.
For instance, could this system have gas giants lurking in its outer reaches, as many other systems with rocky planets (for instance, ours) do? That was the question that a team of scientists, led by researchers from the Carnegie Institute of Science, sought to address in a recent study. According to their findings, TRAPPIST-1 may be orbited by gas giants at a much-greater distance than its seven rocky planets.
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!
