New research led by NASA provides a closer look at a nearby star thought to resemble our young Sun. The work allows scientists to better understand what our Sun may have been like when it was young, and how it may have shaped the atmosphere of our planet and the development of life on Earth.
Many people dream of meeting with a
younger version of themselves to exchange advice, identify the origins of their
defining traits, and share hopes for the future. At 4.65 billion years old, our
Sun is a middle-aged star. Scientists are often curious to learn exactly what
properties enabled our Sun, in its younger years, to support life on nearby
Earth.
Illustration of what the Sun may have been like 4 billion years ago, around the time life developed on Earth. Credits: NASA's Goddard Space Flight Center/Conceptual Image Lab
Without a time machine to transport scientists back billions of years,
retracing our star’s early activity may seem an impossible feat. Luckily, in
the Milky Way galaxy – the glimmering, spiraling segment of the universe where
our solar system is located – there are more than 100 billion stars. One in ten share
characteristics with our Sun, and many are in the early stages of development.
“Imagine I want to reproduce a baby picture of an adult when they were one
or two years old, and all of their pictures were erased or lost. I would look
at a photo of them now, and their close relatives’ photos from around that age,
and from there, reconstruct their baby photos,” said Vladimir Airapetian,
senior astrophysicist in the Heliophysics Division at NASA’s Goddard Space
Flight Center in Greenbelt, Maryland, and first author on the new study.
“That’s the sort of process we are following here – looking at
characteristics of a young star similar to ours, to better understand what our
own star was like in its youth, and what allowed it to foster life on one of
its nearby planets.”
Kappa 1 Ceti is one such solar analogue. The star is located about 30
light-years away (in space terms, that’s like a neighbor who lives on the next
street over) and is estimated to be between 600 to 750 million years old,
around the same age our Sun was when life developed on Earth. It also has a
similar mass and surface temperature to our Sun, said the study’s second
author, Meng Jin, a heliophysicist with the SETI Institute and the Lockheed
Martin Solar and Astrophysics Laboratory in California. All of those
factors make Kappa 1 Ceti a “twin” of our young Sun at the time when life arose
on Earth, and an important target for study.
Airapetian, Jin, and several colleagues have adapted an existing solar
model to predict some of Kappa 1 Ceti’s most important, yet difficult to
measure, characteristics. The model relies on data input from a variety of
space missions including the NASA/ESA Hubble Space Telescope, NASA’s Transiting Exoplanet Survey Satellite and NICER missions, and
ESA’s XMM-Newton. The team
published their study today in The
Astrophysical Journal.
Star Power
Like human toddlers, toddler stars are known for their high bursts of
energy and activity. For stars, one way this pent-up energy is released is in
the form of a stellar wind.
Stellar winds, like stars themselves, are mostly made up of a superhot gas
known as plasma, created when particles in a gas have split into positively
charged ions and negatively charged electrons. The most energetic plasma, with
the help of a star’s magnetic field, can shoot off away from the outermost and
hottest part of a star’s atmosphere, the corona, in an eruption, or stream more
steadily toward nearby planets as stellar wind. “Stellar wind is continuously
flowing out from a star toward its nearby planets, influencing those planets’
environments,” Jin said.
Younger stars tend to generate hotter, more vigorous stellar winds and more
powerful plasma eruptions than older stars do. Such outbursts can affect the
atmosphere and chemistry of planets nearby, and possibly even catalyze the
development of organic material – the building blocks for life – on those
planets.
Stellar wind can have a significant impact on planets at any stage of life.
But the strong, highly dense stellar winds of young stars can compress the
protective magnetic shields of surrounding planets, making them even more
susceptible to the effects of the charged particles.
An artist concept of a coronal mass ejection hitting young Earth's weak magnetosphere. Credits: NASA/GSFC/CIL
Our Sun is a perfect example. Compared to now, in its toddlerhood, our Sun
likely rotated three times faster, had a stronger magnetic field, and shot out
more intense high-energy radiation and particles. These days, for lucky
spectators, the impact of these particles is sometimes visible near the
planet’s poles as aurora, or the Northern and Southern Lights. Airapetian says
4 billion years ago, considering the impact of our Sun’s wind at that time,
these tremendous lights were likely often visible from many more places around
the globe.
That high level of activity in our Sun’s nascence may have pushed back
Earth’s protective magnetosphere, and provided the planet – not close enough to
be torched like Venus, nor distant enough to be neglected like Mars – with the
right atmospheric chemistry for the formation of biological molecules.
Similar processes could be unfolding in stellar systems across our galaxy
and universe.
“It’s my dream to find a rocky exoplanet in the stage that our planet was
in more than 4 billion years ago, being shaped by its young, active star and
nearly ready to host life,” Airapetian said. “Understanding what our Sun was
like just as life was beginning to develop on Earth will help us to refine our
search for stars with exoplanets that may eventually host life.”
A Solar Twin
Though solar analogues can help solve one of the challenges of peeking into
the Sun’s past, time isn’t the only complicating factor in studying our young
Sun. There’s also distance.
We have instruments capable of accurately measuring the stellar wind from
our own Sun, called the solar wind. However, it’s not yet
possible to directly observe the stellar wind of other stars in our galaxy,
like Kappa 1 Ceti, because they are too far away.
When scientists wish to study an event or phenomenon that they cannot
directly observe, scientific modeling can help fill in the gaps. Models are
representations or predictions of an object of study, built on existing
scientific data. While scientists have previously modeled the stellar wind from
this star, Airapetian said, they used more simplified assumptions.
The basis for the new model of Kappa 1 Ceti by Airapetian, Jin, and
colleagues is the Alfvén Wave Solar Model, which is within the Space Weather
Modeling Framework developed by the University of Michigan. The model works by
inputting known information about a star, including its magnetic field and
ultraviolet emission line data, to predict stellar wind activity. When the
model has been tested on our Sun, it has been validated and checked against
observed data to verify that its predictions are accurate.
“It’s capable of modeling our star’s winds and corona with high fidelity,”
Jin said. “And it’s a model we can use on other stars, too, to predict their
stellar wind and thereby investigate habitability. That’s what we did here.”
Previous studies have drawn on data gathered by the Transiting Exoplanet
Survey Satellite (TESS) and Hubble Space Telescope (HST) to identify Kappa 1
Ceti as a young solar proxy, and to gather the necessary inputs for the model,
such as magnetic field and ultraviolet emission line data.
The hot stellar corona, the outermost layer in a star's atmosphere, expands into the stellar wind, driven by heating from the star’s magnetic field and magnetic waves. The researchers modeled the stellar magnetic corona of Kappa 1 Ceti in 3D, based on data from 2012 and 2013. Credits: NASA
“Every model needs input to get output,” Airapetian said. “To get useful,
accurate output, the input needs to be solid data, ideally from multiple
sources across time. We have all that data from Kappa 1 Ceti, but we really
synthesized it in this predictive model to move past previous purely
observational studies of the star.”
Airapetian likens his team’s model to a doctor’s report. To get a full
picture of how a patient is doing, a doctor is likely to talk to the patient,
gather markers like heart rate and temperature, and if needed, conduct several
more specialized tests, like a blood test or ultrasound. They are likely to
formulate an accurate assessment of patient well-being with a combination of
these metrics, not just one.
Similarly, by using many pieces of information about Kappa 1 Ceti gathered
from different space missions, scientists are better able to predict its corona
and the stellar wind. Because stellar wind can affect a nearby planet’s
magnetic shield, it plays an important role in habitability. The team is also
working on another project, looking more closely at the particles that may have
emerged from early solar flares, as well as prebiotic chemistry on Earth.
Our Sun’s Past, Written
in the Stars
The researchers hope to use their model to map the environments of other
Sun-like stars at various life stages.
Specifically, they have eyes on the infant star EK Dra – 111 light-years
away and only 100 million years old – which is likely rotating three times faster
and shooting off more flares and plasma than Kappa 1 Ceti. Documenting how
these similar stars of various ages differ from one another will help
characterize the typical trajectory of a star’s life.
Their work, Airapetian said, is all about “looking at our own Sun, its past
and its possible future, through the lens of other stars.”
To learn more about our Sun's stormy youth, watch this video and see how
energy from our young Sun — 4 billion years ago — aided in creating molecules
in Earth's atmosphere, allowing it to warm up enough to incubate life.
Banner image: A view of the Sun from the Extreme
ultraviolet Imaging Telescope on ESA/NASA's Solar and Heliospheric Observatory,
or SOHO. Credits: ESA/NASA
By Alison Gold
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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