[LETTERHEAD OF SIMPSON THACHER & BARTLETT LLP]

Exhibit 5.1

August 8, 2011

Peabody Energy Corporation
701 Market Street
St. Louis, Missouri 63101

Ladies and Gentlemen:

     We have acted as counsel to Peabody Energy Corporation, a Delaware corporation (the “Company”), in connection with the Registration Statement on Form S-8 (the “Registration Statement”) filed by the Company with the Securities and Exchange Commission under the Securities Act of 1933, as amended, relating to the issuance by the Company of up to an aggregate of 14,000,000 shares of Common Stock, par value $0.01 per share (the “Shares”), pursuant to the Peabody Energy Corporation 2011 Long-Term Equity Incentive Plan (the “Plan”).

     We have examined the Registration Statement, a form of the share certificate and the Plan. We also have examined the originals, or duplicates or certified or conformed copies, of such records, agreements, documents and other instruments and have made such other investigations as we have deemed relevant and necessary in connection with the opinion hereinafter set forth. As to questions of fact material to this opinion, we have relied upon certificates or comparable documents of public officials and of officers and representatives of the Company.

     In rendering the opinion set forth below, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us

Peabody Energy Corporation 2 August 8, 2011

as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies and the authenticity of the originals of such latter documents.

     Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that the issuance of 14,000,000 Shares by the Company pursuant to the Plan has been duly authorized and, upon the issuance and delivery of such Shares in accordance with the Plan, such Shares will be validly issued, fully paid and nonassessable.

     We do not express any opinion herein concerning any law other than the Delaware General Corporation Law (including the statutory provisions, all applicable provisions of the Delaware Constitution and reported judicial decisions interpreting the foregoing).

     We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement.

Very truly yours,
 

/s/ Simpson Thacher & Bartlett LLP  

SIMPSON THACHER & BARTLETT LLP