From the Science Alert: “Astronomers make an incredibly rare discovery of an Earth-like planet 25,000 light years away”
From the Science Alert: “Astronomers make an incredibly rare discovery of an Earth-like planet 25,000 light years away”
From Science Alert
MAY 12, 2020
MICHELLE STARR
Artist’s impression of a super-earth. (ESO / spaceengine.org)
There may be a multitude of Earth-like planets scattered throughout the Milky Way, but they are not that easy to find. To date, only about a third of the over 4,000 exoplanets found and confirmed are rocky – and most of them are within a few thousand light years of Earth.
So the announcement of a new rocky exoplanet is always exciting – but this newly discovered rocky exoplanet is even more exciting.
It belongs to the much smaller subgroup of rocky exoplanets that orbit at Earth-like distance from their star. And it’s a whopping 24,722.65 light-years from us – which could make it the most distant Milky Way exoplanet discovered to date.
It’s so far away that it’s close to – and possibly even within – the galactic bulge, the densely populated region at the center of the galaxy.
Although we’re always better at finding them, exoplanets are tricky little beasts. They don’t give off their own light, and any starlight they could reflect would be a tiny signal that is lost in the noise of their host star.
Most of the exoplanets known to us have been detected using one of two methods. There is the transit method by which planets are recognized by the regular, tiny breaks in starlight when an exoplanet passes in front of them. and there is the wobble method, which detects tiny wobble caused by the gravitational impact of an exoplanet on a star.
However, there is a third method based on the predictions of general relativity: the gravitational microlens. Imagine two stars in a row and an observer (us) some distance away. Rays of light from the rear star (the source) are slightly bent by the gravity of the closer star (the lens) as it passes by. This distorts and enlarges that light source – hence the gravitational microlens.
Gravitation microlens, S. Liebes, Physical Review B, 133 (1964): 835
We know what it looks like with two stars – there are so many that gravitational microlenses are not uncommon. So when an exoplanet is thrown into the mix, it creates another perturbation in the light reaching the viewer; we can see that as the signature of a planet.
Astronomers can then analyze the light curve of the microlens event to determine the parameters of the system.
“To get an idea of the rarity of the discovery, observing the magnification due to the host star took about five days, while the planet was only discovered during a small five-hour distortion,” explained astronomer Antonio Herrera Martin of Canterbury University in New Zealand.
“After confirming that this was actually caused by another ‘body’ different from the star, rather than an instrumental error, we determined the properties of the star-planet system.”
The microlens event – called OGLE-2018-BLG-0677 – was observed independently through two different experiments, the Early Warning System for Optical Gravitational Lens Experiments (OGLE) and the Korea Microlensing Telescope Network (KMTNet). These experiments generally capture about 3,000 microlens events per year, most of which are just stars.
“Dr. Herrera Martin first noticed that the light output of this event had an unusual shape and conducted months of computational analysis that led to the conclusion that this event was due to a star with a low mass planet,” said the astronomer Michael Albrow from the University of Canterbury.
Both sets of data contributed to the team’s analysis.
They discovered that the exoplanet is a super-earth that reaches about 3.96 times the mass of the earth. This makes it one of the lowest mass planets ever discovered using the gravitational microlens.
The star it orbits is really small, only 0.12 times the mass of the Sun – so dainty that researchers couldn’t tell whether it was a low-mass star or a brown dwarf. And the orbital distance between the planet and the star is between 0.63 and 0.72 astronomical units – roughly the distance of Venus from the sun. But because the star is so small, the planet moves around it pretty slowly – its year is roughly 617 days.
We won’t know if the exoplanet might be habitable anytime soon. For one thing, we don’t know the nature of the star. The temperature and activity level of a host star play a big role in habitability as we define it. And the star is so far away that we’re not even anywhere near instruments sensitive enough to examine its spectrum and determine if it has an atmosphere.
However, one of the biggest questions about life in the universe is how often does it have the opportunity to surrender? We know it can arise on rocky exoplanets as it did here on Earth. The more rocky exoplanets we find, the better we can understand this limitation.
What this research shows is the extraordinary power of the gravitational microlens as a tool to find these distant, low mass exoplanets. And it’s damn impressive.
The research was published in the Astronomical Journal.
You can find the full article here.
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