Three new planets found outside solar system could explain why Earth and Neptune are so different

The new planets will allow scientists to study this missing link between rocky Earth-like planets and gas-dominant mini-Neptunes because here all of these types formed in the same system

                            Three new planets found outside solar system could explain why Earth and Neptune are so different

In what has been termed a "rare" finding, three new planets have been discovered outside our Solar System, according to astronomers.

The National Aeronautics and Space Administration's (NASA) Transiting Exoplanet Survey Satellite (TESS), has uncovered three new worlds around a neighboring star that are among the smallest, nearest exoplanets known to date. Of the three new exoplanets — which implies that they are outside our solar system — one is rocky and slightly larger than Earth, while the two others are gaseous and roughly twice Earth’s size.

"The planets orbit a star just 73 light-years away and include a small, rocky super-Earth and two sub-Neptunes, that is, planets about half the size of our own icy giant," states the paper published in Nature Astronomy. 

In our Solar System, there are either small, rocky planets like Earth, Mercury, Venus, and Mars, or much larger planets like Saturn, Jupiter, Uranus, and Neptune that are dominated by gasses rather than land, say scientists. The Earth's Solar System, however, does not have planets about half the size of Neptune, though these are common around other stars. "There is nothing in the Earth's solar system that resembles an intermediate planet, with a size and composition somewhere in the middle of Earth and Neptune," according to the study.

The new star system, which is being called TESS Object of Interest, or TOI-270, is precisely what TESS was designed to find, say researchers. "We have found very few planets like this in the habitable zone, and many fewer around a quiet star, so this is rare. We do not have a planet quite like this in our Solar System," says Stephen Kane, a University of California-Riverside associate professor of planetary astrophysics. Kane adds, "The diameter of our galaxy is 100,000 light-years, and our galaxy is just one of the millions of galaxies. So, 73 light-years mean it's one of our neighboring stars."

What is further interesting, says the research team, is that the sub-Neptune planet that is furthest out from the star appears to be within a "temperate" zone. What this implies, explains the team, is that the very top of the planet's atmosphere is within a temperature range that could support some forms of life. 

"Not only is the smaller planet in the habitable zone — the range of distance from a star that is warm enough to allow liquid-water oceans on a planet — but the TOI-270 star is nearby, making it brighter for viewing. It's also quiet, meaning it has few flares and allows scientists to observe it and its orbiting planets more easily," says the research team.

The astronomers believe TOI-270's sub-Neptunes could be a "missing link" in planetary formation, as they are of intermediate sizes and could help scientists determine whether small, rocky planets like Earth and more massive, icy worlds like Neptune follow the same formation path or evolve separately. "TOI-270 will soon allow us to study this missing link between rocky Earth-like planets and gas-dominant mini-Neptunes because here all of these types formed in the same system," says lead researcher Maximilian Gunther, a Torres postdoctoral fellow at the Massachusetts Institute of Technology (MIT).

NASA's Transiting Exoplanet Survey Satellite, or TESS, discovered three new worlds that are among the smallest, nearest exoplanets (outside Earth’s solar system), known to date. The planets orbit a star just 73 light-years. The new star system, which is being called TESS Object of Interest, or TOI-270, is precisely what TESS was designed to find, say the researchers. (NASA's Goddard Space Flight Center/Scott Wiessinger)

What is TESS?

Launched in 2018, TESS is a NASA Astrophysics Explorer mission, which is led and operated by MIT and managed by NASA's Goddard Space Flight Center. The other partners include Northrop Grumman, NASA's Ames Research Center, the Harvard-Smithsonian Center for Astrophysics, MIT's Lincoln Laboratory, and the Space Telescope Science Institute. More than a dozen universities, research institutes, and observatories worldwide are participants in the mission.

The research team detected the three new planets after looking through measurements of "stellar brightness" taken by TESS. "The satellite stares at patches of the sky for 27 days at a time, monitoring thousands of stars for possible transits, which is "characteristic dips in brightness that could signal a planet temporarily blocking the star's light as it passes in front of it," says the team. 

The team isolated several such signals from a nearby star, located 73 light-years away in the southern sky. They named the star TOI-270 since it is the 270th "TESS Object of Interest" identified to date. "We used ground-based instruments to follow up on the star's activity, and confirmed that the signals are the result of three orbiting exoplanets: planet b, a rocky super-Earth with a roughly three-day orbit; planet c, a sub-Neptune with a five-day orbit; and planet d, another sub-Neptune slightly further out, with an 11-day orbit," according to the paper. 

It further states: "The star is an M-dwarf, a type of star that is normally extremely active, with frequent flares and solar storms. TOI-270 appears to be an older M-dwarf that has since quieted down, giving off a steady brightness, against which scientists can measure many properties of the orbiting planets, such as their mass and atmospheric composition."

The mission of TESS, shown here in a conceptual illustration, is to identify exoplanets orbiting the brightest stars just outside our Solar System. (NASA's Goddard Space Flight Center)

Can the planets support life?

The discovery of TOI-270 had initially caused a lot of excitement among astronomers as it seemed from initial analysis that the planet (sub-Neptune furthest out from the star ) could be in the star's habitable zone, a region that would be "cool enough" for the planet’s surface to support water, and possibly life. However, researchers realized that the smaller planet is unlikely to host life because its surface could be too warm for the presence of liquid water. 

"The planet's atmosphere was probably extremely thick, and would, therefore, generate an intense greenhouse effect, causing the planet’s surface to be too hot to host water or life. But additional planets at greater distances from the star might be cooler, allowing water to pool on their surfaces," state the findings.

The scientists hope further research will reveal additional planets in the system beyond the three discovered. According to them, there is a good possibility that the system hosts other planets, which may lie within the habitable zone. "Planet d, with an 11-day orbit, is about 10 million kilometers out from the star. Given that the star is small and relatively cool, about half as hot as the sun, its habitable zone could potentially begin at around 15 million kilometers. However, whether a planet exists within this zone, and whether it is habitable, depends on multiple parameters such as its size, mass, and atmospheric conditions," say the team. They add that the good news is that the host star, TOI-270 is "remarkably well-suited" for future habitability searches, as it is particularly quiet.

Follow-up observations on the system have been planned for next year when the James Webb Space Telescope launches. It will be able to measure the composition of TOI-270 planets' atmospheres for oxygen, hydrogen, and carbon monoxide. The researchers plan to pin down various properties of the three planets, as well as search for additional planets in the star's habitable zone. These kinds of observations, according to the team, can help determine if a planet has ever had a liquid water ocean and if any of the planets have conditions suitable for life as we know it.

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