Rocking Out With A Heavy Metal Planet

Eike Guenther on the evolving study of planetary origins

17 November 2017
What you’ll discover in this blog post:
  • The diverse range of exoplanets in the Universe and why we study them
  • That stars as dense as lead exist
  • How stars and planets with different compositions are reshaping our ideas of planet formation
Exoplanets are one of the hottest topics in astronomy, and the stuff they’re made of is especially fascinating. Not so long ago, astronomers thought that exoplanets were created from the same interstellar materials as their parent stars, so they must have similar compositions. But in many recently-discovered systems, planets have wildly different compositions to their host stars, turning theories of planet formation on their head. One such planet, named K2-106b, is more than twice as dense as Earth and even denser than lead! Puzzlingly, the parent star of this heavy metal planet has a much lower metallicity than the planet itself. We asked Eike Guenther, the astronomer who led the research on this intriguing system, to fill us in on the details.

Q: In our Solar System we already have eight different planets, so why is it exciting to look elsewhere and study exoplanets?

A: We basically have two species of planets in our Solar System: dense rocky planets that are Earth-sized or smaller, and less-dense gas giants of 15–318 Earth-masses. Rocky planets populate the inner Solar System, while the gas and ice giants orbit in the outer regions. Quite naturally we used to think that all planetary systems would look like our own. However, in the past few decades, we’ve learned that exoplanets are far more diverse than we thought.

We’ve not only found that gas giants can be close to their stars — hot Jupiters — but we also discovered two entirely different species of planets! Super-Jupiters are planets more massive than Jupiter and unexpectedly can have higher densities than the rocky planets in our Solar System. Super-Earths (also known as mini-Neptunes) have masses somewhere between Earth and Neptune, and their densities are surprisingly diverse; some have high densities, others low, and planets of the same mass can actually have very different densities. Without exoplanet research, we never would have known that these classes of planet exist and that planets can be so excitingly varied.


Q: Tell us about your recent work on the planets orbiting the star K2-106. What did you discover?

A: Although exoplanets have hugely diverse properties, we’d always assumed that on the inside, they’re not that different. All planets should have a core, so really the only difference should be the thickness of their hydrogen atmosphere. Using this model, it’s easy to explain the diversity we observe: some planets accrete a lot of hydrogen when they’re young, while others either accrete only very little or lose it as they age.

For the first time, we’d found a “rocky” planet that’s not really rocky — it’s basically just a metal sphere

In our research we focused on ultra-short period planets, which orbit around their stars in less than a day and are exposed to high levels of X-ray and UV radiation that strips away their hydrogen atmospheres. We can try to figure out the composition of the leftover cores by measuring their densities. The first investigations some years ago seemed to suggest that ultra-short period planets have Earth-like compositions, made up of 30% metals and 70% silicates. Then in 2004, the first outlier was discovered — planet 55 Cancri e had a slightly lower density than expected. But the big surprise came this year when we discovered K2-106b. Using radial velocity measurements made with high-resolution spectrographs, including the exoplanet hunter HARPS on the ESO 3.6-metre telescope, we determined the planet’s diameter. From this, we could calculate its density of approximately 13 g/cm3 — huge compared to Earth’s 5.5 g/cm3. This density can only be explained if the planet has an enormous metal content: 80% of its mass is metal! So this heavy metal planet has a completely different composition than the Earth. For the first time, we’d found a “rocky” planet that’s not really rocky — it’s basically just a metal sphere.

Q: But the parent star of K2-106b has a normal metallicity. How could this star and its “heavy metal” planet form from the same cloud of interstellar gas and dust?

A: The planet could have lost its outer envelope of gas in a giant impact, but that’s not so plausible as the planet must have lost about 10 Earth-masses of material. The other possibility is that the planet formed from a material that was already metal-rich.

Since the composition of a protoplanetary disc varies at different distances from a star, we reasoned that K2-106b could have formed in the inner part of the disc. The same conclusion was actually drawn by the MESSENGER mission to Mercury — Mercury didn’t lose its silicate mantle; rather, it formed from a metal-rich material. But Mercury only has 6% of the mass of Earth, while K2-106 is 8 times more massive than Earth. Explaining the formation of such a massive planet in the inner disc is quite a challenge for current theories because in most models there isn’t enough material there for a huge planet to form! However, new observations, especially with ALMA and the Very Large Telescope Interferometer, indicate that material is actually very unevenly distributed in protoplanetary discs, so we have to rethink the assumption that there’s not enough material.

Q: Why is it useful to discover extrasolar planets that look nothing like those we recognise in our Solar System?

A: Prior to the discovery of exoplanets, our models very neatly explained the formation of our Solar System. But after we discovered that exoplanets are so diverse, our theories of planet formation had to be revised. For example, older theories didn’t predict that gas giants could exist close to the host star, but once they were found in other star systems, theories of planet migration were developed. Now with the discovery of K2-106b, we need theories that can also explain how such an extreme object can form.

Studying planets that are unlike those like in our Solar System might also help us understand which planetary ingredients are essential for the formation of life.

Q: In your research, you use very modern tools such data models and simulations, but you also use Kepler’s 400-year-old laws of planetary motion. How much do you think the science of exoplanets has evolved over the last few centuries, or even over your career?


A: It’s fascinating that shortly after the Copernican Revolution people had already begun to speculate about other Earths, changing people’s views about the uniqueness of our planet. For example, the 1686 book Entretiens sur la pluralité des mondes, written by Bernhard le Bovier de Fontenelle, was a dialogue about the plurality of worlds. But what’s changed is that today, we can now address these ideas scientifically. In the last 10 or 20 years, the field of exoplanet research has changed dramatically and unexpected discoveries have played a key role in the direction of our research — for example, the discovery that gas giant planets can exist close to their parent stars led us to the idea of planet migration. Equally exciting were the discoveries of planets like 2M1207b and the HR8799 system, which led to the idea of planet formation via disc fragmentation. It’s always exciting because it’s the unexpected results, not the expected ones, that change our views.

Q: What’s next for your research and for the study of planet formation in general?

A: Our next step in the near future is pretty clear to us. With the upcoming ESPRESSO instrument on ESO’s Very Large Telescope (VLT), we’ll have the chance to determine the masses of planets to a much higher accuracy than ever before, which should tell us if high-density planets like K2-106 are rare or common in the Universe. If the latter is the case, then our theory of planet formation will need to evolve in a new direction.

Over longer timescales, the study of planetary atmospheres will become a cornerstone in the story of planet formation. Exciting studies in this respect have been done with the VLT, but the big leap forward will certainly be the Extremely Large Telescope (ELT), which will allow us to study the atmospheres of smaller planets. Another aspect, which we’re only now beginning to study, is the evolution of planets. It’s truly thrilling to think that the ELT will have the capability to detect a lava planet, like the Earth shortly after it formed. It would be fascinating if we could really “see” how an Earth-like planet actually forms!

Numbers in this article

6%

Mercury’s mass as a fraction of Earth's

5.5

Earth’s density (in grams/cm3)

13

K2-106b’s density (in grams/cm3)

Biography Eike Guenther

Eike Guenther’s future career was sealed from an early age, growing up in an old observatory in Kiel, Germany. He completed his PhD at the Max Planck Institute for Astronomy in Heidelberg and went on to a postdoctoral position at the then-called Queen Mary and Westfield College in London. Eike’s career in the field of exoplanets was kickstarted at the famous “cool-star” conference in Florence, when the discovery of the first brown dwarf and the first exoplanet was announced. He later worked on the space observatory mission CoRot, particularly on the exciting system of CoRoT-7b, and is currently working in the KESPRINT team, studying the diversity of exoplanets using data from the Kepler satellite. Email: guenther@tls-tautenburg.de

Send us your comments!
Subscribe to receive news from ESO in your language
Accelerated by CDN77
Terms & Conditions
Cookie Settings and Policy

Our use of Cookies

We use cookies that are essential for accessing our websites and using our services. We also use cookies to analyse, measure and improve our websites’ performance, to enable content sharing via social media and to display media content hosted on third-party platforms.

You can manage your cookie preferences and find out more by visiting 'Cookie Settings and Policy'.

ESO Cookies Policy


The European Organisation for Astronomical Research in the Southern Hemisphere (ESO) is the pre-eminent intergovernmental science and technology organisation in astronomy. It carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities for astronomy.

This Cookies Policy is intended to provide clarity by outlining the cookies used on the ESO public websites, their functions, the options you have for controlling them, and the ways you can contact us for additional details.

What are cookies?

Cookies are small pieces of data stored on your device by websites you visit. They serve various purposes, such as remembering login credentials and preferences and enhance your browsing experience.

Categories of cookies we use

Essential cookies (always active): These cookies are strictly necessary for the proper functioning of our website. Without these cookies, the website cannot operate correctly, and certain services, such as logging in or accessing secure areas, may not be available; because they are essential for the website’s operation, they cannot be disabled.

Cookie ID/Name
Description/Purpose
Provider (1st party or 3rd party)
Browser session cookie or Stored cookie?
Duration
csrftoken
XSRF protection token. We use this cookie to protect against cross-site request forgery attacks.
1st party
Stored
1 year
user_privacy
Your privacy choices. We use this cookie to save your privacy preferences.
1st party
Stored
6 months
_grecaptcha
We use reCAPTCHA to protect our forms against spam and abuse. reCAPTCHA sets a necessary cookie when executed for the purpose of providing its risk analysis. We use www.recaptcha.net instead of www.google.com in order to avoid unnecessary cookies from Google.
3rd party
Stored
6 months

Functional Cookies: These cookies enhance your browsing experience by enabling additional features and personalization, such as remembering your preferences and settings. While not strictly necessary for the website to function, they improve usability and convenience; these cookies are only placed if you provide your consent.

Cookie ID/Name
Description/Purpose
Provider (1st party or 3rd party)
Browser session cookie or Stored cookie?
Duration
Settings
preferred_language
Language settings. We use this cookie to remember your preferred language settings.
1st party
Stored
1 year
ON | OFF
sessionid
ESO Shop. We use this cookie to store your session information on the ESO Shop. This is just an identifier which is used on the server in order to allow you to purchase items in our shop.
1st party
Stored
2 weeks
ON | OFF

Analytics cookies: These cookies collect information about how visitors interact with our website, such as which pages are visited most often and how users navigate the site. This data helps us improve website performance, optimize content, and enhance the user experience; these cookies are only placed if you provide your consent. We use the following analytics cookies.

Matomo Cookies:

This website uses Matomo (formerly Piwik), an open source software which enables the statistical analysis of website visits. Matomo uses cookies (text files) which are saved on your computer and which allow us to analyze how you use our website. The website user information generated by the cookies will only be saved on the servers of our IT Department. We use this information to analyze www.eso.org visits and to prepare reports on website activities. These data will not be disclosed to third parties.

On behalf of ESO, Matomo will use this information for the purpose of evaluating your use of the website, compiling reports on website activity and providing other services relating to website activity and internet usage.

ON | OFF

Matomo cookies settings:

Cookie ID/Name
Description/Purpose
Provider (1st party or 3rd party)
Browser session cookie or Stored cookie?
Duration
Settings
_pk_id
Stores a unique visitor ID.
1st party
Stored
13 months
_pk_ses
Session cookie temporarily stores data for the visit.
1st party
Stored
30 minutes
_pk_ref
Stores attribution information (the referrer that brought the visitor to the website).
1st party
Stored
6 months
_pk_testcookie
Temporary cookie to check if a visitor’s browser supports cookies (set in Internet Explorer only).
1st party
Stored
Temporary cookie that expires almost immediately after being set.

Additional Third-party cookies on ESO websites: some of our pages display content from external providers, e.g. YouTube.

Such third-party services are outside of ESO control and may, at any time, change their terms of service, use of cookies, etc.

YouTube: Some videos on the ESO website are embedded from ESO’s official YouTube channel. We have enabled YouTube’s privacy-enhanced mode, meaning that no cookies are set unless the user actively clicks on the video to play it. Additionally, in this mode, YouTube does not store any personally identifiable cookie data for embedded video playbacks. For more details, please refer to YouTube’s embedding videos information page.

Cookies can also be classified based on the following elements.

Regarding the domain, there are:

As for their duration, cookies can be:

How to manage cookies

Cookie settings: You can modify your cookie choices for the ESO webpages at any time by clicking on the link Cookie settings at the bottom of any page.

In your browser: If you wish to delete cookies or instruct your browser to delete or block cookies by default, please visit the help pages of your browser:

Please be aware that if you delete or decline cookies, certain functionalities of our website may be not be available and your browsing experience may be affected.

You can set most browsers to prevent any cookies being placed on your device, but you may then have to manually adjust some preferences every time you visit a site/page. And some services and functionalities may not work properly at all (e.g. profile logging-in, shop check out).

Updates to the ESO Cookies Policy

The ESO Cookies Policy may be subject to future updates, which will be made available on this page.

Additional information

For any queries related to cookies, please contact: pdprATesoDOTorg.

As ESO public webpages are managed by our Department of Communication, your questions will be dealt with the support of the said Department.