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takyon writes:

Astronomers have observed enough planetary transits to confirm the existence of seven "Earth-sized" exoplanets orbiting TRAPPIST-1, an ultra-cool (~2550 K) red dwarf star about 39.5 light years away. Three of the exoplanets are located inside the "habitable zone" of their parent star. These three orbit from 0.028 to 0.045 AU away from the star:

Astronomers using the TRAPPIST–South telescope at ESO's La Silla Observatory, the Very Large Telescope (VLT) at Paranal and the NASA Spitzer Space Telescope, as well as other telescopes around the world, have now confirmed the existence of at least seven small planets orbiting the cool red dwarf star TRAPPIST-1. All the planets, labelled TRAPPIST-1b, c, d, e, f, g and h in order of increasing distance from their parent star, have sizes similar to Earth.

The exoplanets are presumed to be tidally locked. The six closest to TRAPPIST-1 have been determined to be rocky, while the seventh, TRAPPIST-1h, requires additional observations to determine its characteristics due to its longer orbital period.

Mass estimates for the planets range from 0.41 Earth masses (M_⊕) to 1.38 M_⊕. Radii range from 0.76 Earth radii (R_⊕) to 1.13 R_⊕.

Spitzer, Hubble, and other telescopes will continue to make observations of the TRAPPIST-1 planetary system, but the best data will likely come from the James Webb Space Telescope (JWST), which is scheduled to launch in late 2018. JWST will allow the atmospheres and temperatures of many exoplanets to be characterized, which will help to settle whether the "habitable zones" of red dwarf stars are actually hospitable.

Artist illustrations and data for the TRAPPIST-1 system compared to Mercury, Venus, Mars, and Earth.

Here's a website dedicated to the star.

Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1 (DOI: 10.1038/nature21360) (DX)

Original Submission

takyon writes:

Another study has cast doubt on the habitability of an Earth-like exoplanet in the "habitable zone" of a red dwarf, in this case Proxima Centauri specifically:

At only four light-years away, Proxima b is our closest known extra-solar neighbor. However, due to the fact that it hasn't been seen crossing in front of its host star, the exoplanet eludes the usual method for learning about its atmosphere. Instead, scientists must rely on models to understand whether the exoplanet is habitable.

One such computer model considered what would happen if Earth orbited Proxima Centauri, our nearest stellar neighbor and Proxima b's host star, at the same orbit as Proxima b. The NASA study, published on July 24, 2017, in The Astrophysical Journal Letters [DOI: 10.3847/2041-8213/aa7eca], suggests Earth's atmosphere wouldn't survive in close proximity to the violent red dwarf.

[...] In Proxima Centauri's habitable zone, Proxima b encounters bouts of extreme ultraviolet radiation hundreds of times greater than Earth does from the sun. That radiation generates enough energy to strip away not just the lightest molecules — hydrogen — but also, over time, heavier elements such as oxygen and nitrogen.

The model shows Proxima Centauri's powerful radiation drains the Earth-like atmosphere as much as 10,000 times faster than what happens at Earth.

Previously: "Earth-Like" Exoplanet Found in Habitable Zone of Proxima Centauri
Proxima b May Have Oceans
Researchers Suffocate Hopes of Life Support in Red Dwarf "Habitable Zones"
Proxima B Habitability Study Adds Climate Model

Related: MAVEN Results Find Solar Wind and Radiation Responsible for Stripping the Martian Atmosphere

Original Submission

takyon writes:

The presence of large quantities of oxygen ions may be able to distinguish habitable exoplanets with life from barren exoplanets in the habitable zone (resembling Venus or Mars):

Like Earth, Venus and Mars are small rocky planets; they have permanent atmospheres like Earth, and their atmospheres are exposed to the same solar radiation as Earth's. Data from the Pioneer Venus Orbiter and the Viking descent probe on Mars show that they have very similar ionospheres to each other—which don't contain a lot of atomic O⁺ ions. Know what else Venus and Mars are missing? Photosynthesis.

[Astronomy PhD candidate Paul] Dalba's contention is that photosynthesis on a planet's surface, which generates a surfeit of molecular oxygen, is the only thing that can account for these atomic O⁺ ions in a planet's ionosphere. The mere existence of life throws a planet's atmosphere out of chemical balance. O⁺ would be a neat biomarker because there isn't a numerical cutoff required—just the dominance of O+ among the ionic species in the upper atmosphere would indicate "thriving global biological activity" on the planet below.

Dalba claims that Venus and Mars act as negative controls, demonstrating that planets like Earth but lacking life don't have this O+ layer. Some may think that continuous volcanic activity on the surface could also generate enough oxygen, but Dalba doesn't. Chemistry involving water and UV light [open, DOI: 10.1038/srep13977] [DX] can also release oxygen. But the amount of water on Earth is insufficient to account for the requisite oxygen content, so he thinks that the presence of water on other planets wouldn't make enough oxygen there either.

Atomic oxygen ions as ionospheric biomarkers on exoplanets (DOI: 10.1038/s41550-017-0375-y) (DX)

Related: Nitrogen in Ancient Rocks a Sign of Early Life
Oxygen Ions From Earth Escape to the Moon
Researchers Suffocate Hopes of Life Support in Red Dwarf "Habitable Zones"
Seven Earth-Sized Exoplanets, Including Three Potentially Habitable, Identified Around TRAPPIST-1
Cosmic Methyl Chloride Detection Complicates the Search for Life on Exoplanets
Mars Colonists Could Produce Oxygen by Making a Plasma Out of Atmospheric Carbon Dioxide
Analysis of Microfossils Finds that Microbial Life Existed at Least 3.5 Billion Years Ago
To Detect Life on Other Planets, Look for Methane, Carbon Dioxide, and an Absence of Carbon Monoxide

Original Submission