Astronomers discover smallest exoplanet ever with water vapor

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View largerThis blue arc is an artist’s conception of a water-world.. | That blue arc at the top is an artist’s concept of a water world – GJ 9827 – the smallest exoplanet yet with water vapor in its atmosphere. It’s 97 light-years away. Image via NASA/ Hubblesite/ ESA/ Leah Hustak (STScI)/ Ralf Crawford (STScI).

Smallest exoplanet with water vapor

Water vapor has been found in the atmospheres and other larger bodies by astronomers. Gas giant exoplanets, orbiting distant suns. But on January 25, 2024 they will be discovered. You can also read about the importance of this in our article they’ve found water vapor on the smallest exoplanet yet. GJ 9827d is only twice Earth’s diameter. It’s 97 light-yearsThe constellation is a good direction to go. Pisces. Why are we so interested? Because we’re curious about the possibility of other Earths.

We know GJ 9827 d itself isn’t habitable. It’s inhospitable for life as we know it. It’s a searing hot world, with surface temperatures of about 800 degrees Fahrenheit (430 C). That’s about as hot as as the planet Venus in our solar system. It’s hot enough to melt lead!

Researchers first Published by:The following are some examples of how to get started: peer-reviewedPaper The Astrophysical Journal LettersOn September 12, 2023.

They said that the discovery could mean smaller worlds with lots of water are not uncommon.

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Water vapor is present in the atmosphere of a hot, steamy exoplanet

The mighty planet-hunting KeplerIn 2017, a space telescope discovered GJ9827 d. This world orbits around its Red dwarf starIn just 6.2 days. And it’s now the smallest exoplanet where water vapor has been found in its atmosphere. One of the study’s lead authors, Björn BennekeThe Trottier institute for research on exoplanets is located at the University of Montreal (Canada). You can also read about the importance of this in our article:

This would be the very first time we could directly demonstrate through an atmospheric detection, that these planets can exist around other stars. This is an important first step towards determining the presence and diversity of atmospherics on rocky planetary systems.

The discovery is a step in the right direction to find rocky exoplanets with water on their surface. As co-principal investigator Laura KreidbergThe Max Planck Institute for Astronomy, located in Heidelberg, Germany, has added:

A planet so small with water is a significant discovery. It is closer than ever before to defining truly Earth-like planets.

Hubble observed GJ9827 d in 11 TransitsOver three years. As seen from Earth, the planet crossed infront of its star. We don’t know yet if the planet has clouds in its atmosphere. If there are any clouds, then they must be at a low altitude. Otherwise, they would block Hubble’s view of the atmosphere itself.

4 planets, going from smallest to largest, left to right. White text label below each one, on black background.
View largerThis image is a size comparison between 2 sub-Neptune. | This image depicts a size comparison of 2 sub-Neptune ExoplanetsEarth and Neptune, TOI-421b, and GJ 1214b. TOI-421 b is a hot sub-Neptune with a radius 2.68 times Earth’s and a mass 7.2 times Earth’s. GJ 1214 b, meanwhile, is a warm sub-Neptune with a radius 2.74 times Earth’s and a mass 8.2 times Earth’s. Astronomers use the two terms Sub-Neptune, mini-NeptuneThis can be confusing. Generally speaking, mini-Neptunes are smaller than Neptune and have thick hydrogen–helium atmospheres. Sub-Neptunes can be any planet with a smaller radius, but still have a higher mass, including mini Neptunes. Image via NASA/ Webb Telescope/ ESA/ CSA/ Dani Player STScI.

Rich water vapor or mini-Neptune

There are still many questions in the case of GJ9827 d. Even with the new Hubble results, scientists aren’t sure if the small exoplanet has a steamy atmosphere of mostly water vapor, or a hydrogen atmosphere with just a little water vapor. If it’s the latter, then it would be like a mini-NeptuneIt is more than a rocky Earth type world. Lead author Pierre-Alexis RoyThe University of Montreal stated that it is exciting for astronomers in either case:

Our observing program, led by principal investigator Ian Crossfield of Kansas University in Lawrence, Kansas, was designed specifically with the goal to not only detect the molecules in the planet’s atmosphere, but to actually look specifically for water vapor. Water vapor could be dominant or only a small species in an atmosphere dominated by hydrogen.

Researchers note that of the two scenarios they say the water world scenario seems to be the most likely. The paper stated:

Given that recent studies have inferred an important mass-loss rate (>0.5 M. Gyr^1) for GJ 9827 d, making it unlikely to retain a H-dominated envelope, our findings highlight GJ 9827 d as a promising water world candidate that could host a volatile-dominated atmosphere.

A warmer version Europa?

If the planet is rocky, then it could be like a warmer version of Jupiter’s moon Europa, the researchers said. Benneke said:

The planet GJ9827d could be half rock, half water. Water vapor would be present on top of the smaller rocky bodies.

If this is the case, then it is likely that the planet formed first farther away from its original star. Ice would have been the water form. GJ 9827d migrated further inwards, to where it now is. In this scenario most or all hydrogen would have been burned off. This would leave an atmosphere dominated by water vapor.

Shiny silver cylindrical spacecraft with large solar panels floating above Earth.
NASA’s Hubble Space TelescopeThe water vapor in atmosphere was discovered GJ 9827. Image via NASA.

Clues to other small exoplanets

The observations provide valuable clues about smaller planets. And NASA’s WebbHubble is not able to analyze the atmospheres of some planets with as much detail as a space telescope. Thomas Greene at NASA’s Ames Research Center in California said:

Observing the water in a body of water can open up a world of possibilities. This Hubble discovery opens up the possibility of future studies of these types of worlds by the James Webb Space Telescope. JWST can see a lot more with additional infrared measurements, including carbon-bearing compounds like carbon monoxide and carbon dioxide. Once we get a total inventory of a planet’s elements, we can compare those to the star it orbits and understand how it was formed.

It’s a huge breakthrough that we can detect atmospheres even on these small planets. Benneke said:

Up until now, we were unable to detect the atmospheric pressure of such a tiny planet. And we’re slowly getting in this regime now. At some point, as we study smaller planets, there must be a transition where there’s no more hydrogen on these small worlds, and they have atmospheres more like Venus (which is dominated by carbon dioxide).

The debate over water world versus mini-Neptune reminds us of another controversy involving Exoplanet LHS1140bIt is also likely to be either a water-world (but with oceans), or a miniature Neptune. It is only about 50 light-years away from Earth.

Bottom line: Astronomers found the smallest yet exoplanet with water vapor. GJ 9827d is a hot, steamy world. But is it rocky as Earth or a miniature Neptune?

Source: Water Absorption within the Transmission Spectrum for the Water World Candidate, GJ 9827

Via Hubblesite

Read more: Is the nearby exoplanet an ocean world? Or a mini Neptune?

Read more: Nearby Exoplanet Is Earth’s Twin, But Hot as a Pizza Oven

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